xnu-6153.11.26.tar.gz

[apple/xnu.git] / osfmk / kern / gzalloc.c

/*
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 *
 * @APPLE_OSREFERENCE_LICENSE_HEADER_START@
 *
 * This file contains Original Code and/or Modifications of Original Code
 * as defined in and that are subject to the Apple Public Source License
 * Version 2.0 (the 'License'). You may not use this file except in
 * compliance with the License. The rights granted to you under the License
 * may not be used to create, or enable the creation or redistribution of,
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 * Please obtain a copy of the License at
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/*
 *      File:   kern/gzalloc.c
 *      Author: Derek Kumar
 *
 *      "Guard mode" zone allocator, used to trap use-after-free errors,
 *      overruns, underruns, mismatched allocations/frees, uninitialized
 *      zone element use, timing dependent races etc.
 *
 *      The allocator is configured by these boot-args:
 *      gzalloc_size=<size>: target all zones with elements of <size> bytes
 *      gzalloc_min=<size>: target zones with elements >= size
 *      gzalloc_max=<size>: target zones with elements <= size
 *      gzalloc_min/max can be specified in conjunction to target a range of
 *      sizes
 *      gzalloc_fc_size=<size>: number of zone elements (effectively page
 *      multiple sized) to retain in the free VA cache. This cache is evicted
 *      (backing pages and VA released) in a least-recently-freed fashion.
 *      Larger free VA caches allow for a longer window of opportunity to trap
 *      delayed use-after-free operations, but use more memory.
 *      -gzalloc_wp: Write protect, rather than unmap, freed allocations
 *      lingering in the free VA cache. Useful to disambiguate between
 *      read-after-frees/read overruns and writes. Also permits direct inspection
 *      of the freed element in the cache via the kernel debugger. As each
 *      element has a "header" (trailer in underflow detection mode), the zone
 *      of origin of the element can be easily determined in this mode.
 *      -gzalloc_uf_mode: Underflow detection mode, where the guard page
 *      adjoining each element is placed *before* the element page rather than
 *      after. The element is also located at the top of the page, rather than
 *      abutting the bottom as with the standard overflow detection mode.
 *      -gzalloc_noconsistency: disable consistency checks that flag mismatched
 *      frees, corruptions of the header/trailer signatures etc.
 *      -nogzalloc_mode: Disables the guard mode allocator. The DEBUG kernel
 *      enables the guard allocator for zones sized 1K (if present) by
 *      default, this option can disable that behaviour.
 *      gzname=<name> target a zone by name. Can be coupled with size-based
 *      targeting. Naming conventions match those of the zlog boot-arg, i.e.
 *      "a period in the logname will match a space in the zone name"
 *      -gzalloc_no_dfree_check Eliminate double free checks
 *      gzalloc_zscale=<value> specify size multiplier for the dedicated gzalloc submap
 */

#include <zone_debug.h>

#include <mach/mach_types.h>
#include <mach/vm_param.h>
#include <mach/kern_return.h>
#include <mach/machine/vm_types.h>
#include <mach_debug/zone_info.h>
#include <mach/vm_map.h>

#include <kern/kern_types.h>
#include <kern/assert.h>
#include <kern/sched.h>
#include <kern/locks.h>
#include <kern/misc_protos.h>
#include <kern/zalloc.h>
#include <kern/kalloc.h>

#include <vm/pmap.h>
#include <vm/vm_map.h>
#include <vm/vm_kern.h>
#include <vm/vm_page.h>

#include <pexpert/pexpert.h>

#include <machine/machparam.h>

#include <libkern/OSDebug.h>
#include <libkern/OSAtomic.h>
#include <sys/kdebug.h>

extern boolean_t vm_kernel_ready, kmem_ready;
boolean_t gzalloc_mode = FALSE;
uint32_t pdzalloc_count, pdzfree_count;

#define GZALLOC_MIN_DEFAULT (1024)
#define GZDEADZONE ((zone_t) 0xDEAD201E)
#define GZALLOC_SIGNATURE (0xABADCAFE)
#define GZALLOC_RESERVE_SIZE_DEFAULT (2 * 1024 * 1024)
#define GZFC_DEFAULT_SIZE (1536)

char gzalloc_fill_pattern = 0x67; /* 'g' */

uint32_t gzalloc_min = ~0U;
uint32_t gzalloc_max = 0;
uint32_t gzalloc_size = 0;
uint64_t gzalloc_allocated, gzalloc_freed, gzalloc_early_alloc, gzalloc_early_free, gzalloc_wasted;
boolean_t gzalloc_uf_mode = FALSE, gzalloc_consistency_checks = TRUE, gzalloc_dfree_check = TRUE;
vm_prot_t gzalloc_prot = VM_PROT_NONE;
uint32_t gzalloc_guard = KMA_GUARD_LAST;
uint32_t gzfc_size = GZFC_DEFAULT_SIZE;
uint32_t gzalloc_zonemap_scale = 6;

vm_map_t gzalloc_map;
vm_offset_t gzalloc_map_min, gzalloc_map_max;
vm_offset_t gzalloc_reserve;
vm_size_t gzalloc_reserve_size;

typedef struct gzalloc_header {
        zone_t gzone;
        uint32_t  gzsize;
        uint32_t  gzsig;
} gzhdr_t;

#define GZHEADER_SIZE (sizeof(gzhdr_t))

extern zone_t vm_page_zone;

static zone_t gztrackzone = NULL;
static char gznamedzone[MAX_ZONE_NAME] = "";

void
gzalloc_reconfigure(__unused zone_t z)
{
        /* Nothing for now */
}

boolean_t
gzalloc_enabled(void)
{
        return gzalloc_mode;
}

static inline boolean_t
gzalloc_tracked(zone_t z)
{
        return gzalloc_mode &&
               (((z->elem_size >= gzalloc_min) && (z->elem_size <= gzalloc_max)) || (z == gztrackzone)) &&
               (z->gzalloc_exempt == 0);
}

void
gzalloc_zone_init(zone_t z)
{
        if (gzalloc_mode) {
                bzero(&z->gz, sizeof(z->gz));

                if (track_this_zone(z->zone_name, gznamedzone)) {
                        gztrackzone = z;
                }

                if (gzfc_size &&
                    gzalloc_tracked(z)) {
                        vm_size_t gzfcsz = round_page(sizeof(*z->gz.gzfc) * gzfc_size);

                        /* If the VM/kmem system aren't yet configured, carve
                         * out the free element cache structure directly from the
                         * gzalloc_reserve supplied by the pmap layer.
                         */
                        if (!kmem_ready) {
                                if (gzalloc_reserve_size < gzfcsz) {
                                        panic("gzalloc reserve exhausted");
                                }

                                z->gz.gzfc = (vm_offset_t *)gzalloc_reserve;
                                gzalloc_reserve += gzfcsz;
                                gzalloc_reserve_size -= gzfcsz;
                        } else {
                                kern_return_t kr;

                                if ((kr = kernel_memory_allocate(kernel_map, (vm_offset_t *)&z->gz.gzfc, gzfcsz, 0, KMA_KOBJECT, VM_KERN_MEMORY_OSFMK)) != KERN_SUCCESS) {
                                        panic("zinit/gzalloc: kernel_memory_allocate failed (%d) for 0x%lx bytes", kr, (unsigned long) gzfcsz);
                                }
                        }
                        bzero((void *)z->gz.gzfc, gzfcsz);
                }
        }
}

/* Called by zdestroy() to dump the free cache elements so the zone count can drop to zero. */
void
gzalloc_empty_free_cache(zone_t zone)
{
        if (__improbable(gzalloc_tracked(zone))) {
                kern_return_t kr;
                int freed_elements = 0;
                vm_offset_t free_addr = 0;
                vm_offset_t rounded_size = round_page(zone->elem_size + GZHEADER_SIZE);
                vm_offset_t gzfcsz = round_page(sizeof(*zone->gz.gzfc) * gzfc_size);
                vm_offset_t gzfc_copy;

                kr = kmem_alloc(kernel_map, &gzfc_copy, gzfcsz, VM_KERN_MEMORY_OSFMK);
                if (kr != KERN_SUCCESS) {
                        panic("gzalloc_empty_free_cache: kmem_alloc: 0x%x", kr);
                }

                /* Reset gzalloc_data. */
                lock_zone(zone);
                memcpy((void *)gzfc_copy, (void *)zone->gz.gzfc, gzfcsz);
                bzero((void *)zone->gz.gzfc, gzfcsz);
                zone->gz.gzfc_index = 0;
                unlock_zone(zone);

                /* Free up all the cached elements. */
                for (uint32_t index = 0; index < gzfc_size; index++) {
                        free_addr = ((vm_offset_t *)gzfc_copy)[index];
                        if (free_addr && free_addr >= gzalloc_map_min && free_addr < gzalloc_map_max) {
                                kr = vm_map_remove(
                                        gzalloc_map,
                                        free_addr,
                                        free_addr + rounded_size + (1 * PAGE_SIZE),
                                        VM_MAP_REMOVE_KUNWIRE);
                                if (kr != KERN_SUCCESS) {
                                        panic("gzalloc_empty_free_cache: vm_map_remove: %p, 0x%x", (void *)free_addr, kr);
                                }
                                OSAddAtomic64((SInt32)rounded_size, &gzalloc_freed);
                                OSAddAtomic64(-((SInt32) (rounded_size - zone->elem_size)), &gzalloc_wasted);

                                freed_elements++;
                        }
                }
                /*
                 * TODO: Consider freeing up zone->gz.gzfc as well if it didn't come from the gzalloc_reserve pool.
                 * For now we're reusing this buffer across zdestroy's. We would have to allocate it again on a
                 * subsequent zinit() as well.
                 */

                /* Decrement zone counters. */
                lock_zone(zone);
                zone->count -= freed_elements;
                zone->cur_size -= (freed_elements * rounded_size);
                unlock_zone(zone);

                kmem_free(kernel_map, gzfc_copy, gzfcsz);
        }
}

void
gzalloc_configure(void)
{
        char temp_buf[16];

        if (PE_parse_boot_argn("-gzalloc_mode", temp_buf, sizeof(temp_buf))) {
                gzalloc_mode = TRUE;
                gzalloc_min = GZALLOC_MIN_DEFAULT;
                gzalloc_max = ~0U;
        }

        if (PE_parse_boot_argn("gzalloc_min", &gzalloc_min, sizeof(gzalloc_min))) {
                gzalloc_mode = TRUE;
                gzalloc_max = ~0U;
        }

        if (PE_parse_boot_argn("gzalloc_max", &gzalloc_max, sizeof(gzalloc_max))) {
                gzalloc_mode = TRUE;
                if (gzalloc_min == ~0U) {
                        gzalloc_min = 0;
                }
        }

        if (PE_parse_boot_argn("gzalloc_size", &gzalloc_size, sizeof(gzalloc_size))) {
                gzalloc_min = gzalloc_max = gzalloc_size;
                gzalloc_mode = TRUE;
        }

        (void)PE_parse_boot_argn("gzalloc_fc_size", &gzfc_size, sizeof(gzfc_size));

        if (PE_parse_boot_argn("-gzalloc_wp", temp_buf, sizeof(temp_buf))) {
                gzalloc_prot = VM_PROT_READ;
        }

        if (PE_parse_boot_argn("-gzalloc_uf_mode", temp_buf, sizeof(temp_buf))) {
                gzalloc_uf_mode = TRUE;
                gzalloc_guard = KMA_GUARD_FIRST;
        }

        if (PE_parse_boot_argn("-gzalloc_no_dfree_check", temp_buf, sizeof(temp_buf))) {
                gzalloc_dfree_check = FALSE;
        }

        (void) PE_parse_boot_argn("gzalloc_zscale", &gzalloc_zonemap_scale, sizeof(gzalloc_zonemap_scale));

        if (PE_parse_boot_argn("-gzalloc_noconsistency", temp_buf, sizeof(temp_buf))) {
                gzalloc_consistency_checks = FALSE;
        }

        if (PE_parse_boot_argn("gzname", gznamedzone, sizeof(gznamedzone))) {
                gzalloc_mode = TRUE;
        }
#if DEBUG
        if (gzalloc_mode == FALSE) {
                gzalloc_min = 1024;
                gzalloc_max = 1024;
                strlcpy(gznamedzone, "pmap", sizeof(gznamedzone));
                gzalloc_prot = VM_PROT_READ;
                gzalloc_mode = TRUE;
        }
#endif
        if (PE_parse_boot_argn("-nogzalloc_mode", temp_buf, sizeof(temp_buf))) {
                gzalloc_mode = FALSE;
        }

        if (gzalloc_mode) {
                gzalloc_reserve_size = GZALLOC_RESERVE_SIZE_DEFAULT;
                gzalloc_reserve = (vm_offset_t) pmap_steal_memory(gzalloc_reserve_size);
        }
}

void
gzalloc_init(vm_size_t max_zonemap_size)
{
        kern_return_t retval;

        if (gzalloc_mode) {
                vm_map_kernel_flags_t vmk_flags;

                vmk_flags = VM_MAP_KERNEL_FLAGS_NONE;
                vmk_flags.vmkf_permanent = TRUE;
                retval = kmem_suballoc(kernel_map, &gzalloc_map_min, (max_zonemap_size * gzalloc_zonemap_scale),
                    FALSE, VM_FLAGS_ANYWHERE, vmk_flags, VM_KERN_MEMORY_ZONE,
                    &gzalloc_map);

                if (retval != KERN_SUCCESS) {
                        panic("zone_init: kmem_suballoc(gzalloc_map, 0x%lx, %u) failed", max_zonemap_size, gzalloc_zonemap_scale);
                }
                gzalloc_map_max = gzalloc_map_min + (max_zonemap_size * gzalloc_zonemap_scale);
        }
}

vm_offset_t
gzalloc_alloc(zone_t zone, boolean_t canblock)
{
        vm_offset_t addr = 0;

        if (__improbable(gzalloc_tracked(zone))) {
                if (get_preemption_level() != 0) {
                        if (canblock == TRUE) {
                                pdzalloc_count++;
                        } else {
                                return 0;
                        }
                }

                vm_offset_t rounded_size = round_page(zone->elem_size + GZHEADER_SIZE);
                vm_offset_t residue = rounded_size - zone->elem_size;
                vm_offset_t gzaddr = 0;
                gzhdr_t *gzh, *gzhcopy = NULL;

                if (!kmem_ready || (vm_page_zone == ZONE_NULL)) {
                        /* Early allocations are supplied directly from the
                         * reserve.
                         */
                        if (gzalloc_reserve_size < (rounded_size + PAGE_SIZE)) {
                                panic("gzalloc reserve exhausted");
                        }
                        gzaddr = gzalloc_reserve;
                        /* No guard page for these early allocations, just
                         * waste an additional page.
                         */
                        gzalloc_reserve += rounded_size + PAGE_SIZE;
                        gzalloc_reserve_size -= rounded_size + PAGE_SIZE;
                        OSAddAtomic64((SInt32) (rounded_size), &gzalloc_early_alloc);
                } else {
                        kern_return_t kr = kernel_memory_allocate(gzalloc_map,
                            &gzaddr, rounded_size + (1 * PAGE_SIZE),
                            0, KMA_KOBJECT | KMA_ATOMIC | gzalloc_guard,
                            VM_KERN_MEMORY_OSFMK);
                        if (kr != KERN_SUCCESS) {
                                panic("gzalloc: kernel_memory_allocate for size 0x%llx failed with %d", (uint64_t)rounded_size, kr);
                        }
                }

                if (gzalloc_uf_mode) {
                        gzaddr += PAGE_SIZE;
                        /* The "header" becomes a "footer" in underflow
                         * mode.
                         */
                        gzh = (gzhdr_t *) (gzaddr + zone->elem_size);
                        addr = gzaddr;
                        gzhcopy = (gzhdr_t *) (gzaddr + rounded_size - sizeof(gzhdr_t));
                } else {
                        gzh = (gzhdr_t *) (gzaddr + residue - GZHEADER_SIZE);
                        addr = (gzaddr + residue);
                }

                /* Fill with a pattern on allocation to trap uninitialized
                 * data use. Since the element size may be "rounded up"
                 * by higher layers such as the kalloc layer, this may
                 * also identify overruns between the originally requested
                 * size and the rounded size via visual inspection.
                 * TBD: plumb through the originally requested size,
                 * prior to rounding by kalloc/IOMalloc etc.
                 * We also add a signature and the zone of origin in a header
                 * prefixed to the allocation.
                 */
                memset((void *)gzaddr, gzalloc_fill_pattern, rounded_size);

                gzh->gzone = (kmem_ready && vm_page_zone) ? zone : GZDEADZONE;
                gzh->gzsize = (uint32_t) zone->elem_size;
                gzh->gzsig = GZALLOC_SIGNATURE;

                /* In underflow detection mode, stash away a copy of the
                 * metadata at the edge of the allocated range, for
                 * retrieval by gzalloc_element_size()
                 */
                if (gzhcopy) {
                        *gzhcopy = *gzh;
                }

                lock_zone(zone);
                assert(zone->zone_valid);
                zone->count++;
                zone->sum_count++;
                zone->cur_size += rounded_size;
                unlock_zone(zone);

                OSAddAtomic64((SInt32) rounded_size, &gzalloc_allocated);
                OSAddAtomic64((SInt32) (rounded_size - zone->elem_size), &gzalloc_wasted);
        }
        return addr;
}

boolean_t
gzalloc_free(zone_t zone, void *addr)
{
        boolean_t gzfreed = FALSE;
        kern_return_t kr;

        if (__improbable(gzalloc_tracked(zone))) {
                gzhdr_t *gzh;
                vm_offset_t rounded_size = round_page(zone->elem_size + GZHEADER_SIZE);
                vm_offset_t residue = rounded_size - zone->elem_size;
                vm_offset_t saddr;
                vm_offset_t free_addr = 0;

                if (gzalloc_uf_mode) {
                        gzh = (gzhdr_t *)((vm_offset_t)addr + zone->elem_size);
                        saddr = (vm_offset_t) addr - PAGE_SIZE;
                } else {
                        gzh = (gzhdr_t *)((vm_offset_t)addr - GZHEADER_SIZE);
                        saddr = ((vm_offset_t)addr) - residue;
                }

                if ((saddr & PAGE_MASK) != 0) {
                        panic("gzalloc_free: invalid address supplied: %p (adjusted: 0x%lx) for zone with element sized 0x%lx\n", addr, saddr, zone->elem_size);
                }

                if (gzfc_size) {
                        if (gzalloc_dfree_check) {
                                uint32_t gd;

                                lock_zone(zone);
                                assert(zone->zone_valid);
                                for (gd = 0; gd < gzfc_size; gd++) {
                                        if (zone->gz.gzfc[gd] == saddr) {
                                                panic("gzalloc: double free detected, freed address: 0x%lx, current free cache index: %d, freed index: %d", saddr, zone->gz.gzfc_index, gd);
                                        }
                                }
                                unlock_zone(zone);
                        }
                }

                if (gzalloc_consistency_checks) {
                        if (gzh->gzsig != GZALLOC_SIGNATURE) {
                                panic("GZALLOC signature mismatch for element %p, expected 0x%x, found 0x%x", addr, GZALLOC_SIGNATURE, gzh->gzsig);
                        }

                        if (gzh->gzone != zone && (gzh->gzone != GZDEADZONE)) {
                                panic("%s: Mismatched zone or under/overflow, current zone: %p, recorded zone: %p, address: %p", __FUNCTION__, zone, gzh->gzone, (void *)addr);
                        }
                        /* Partially redundant given the zone check, but may flag header corruption */
                        if (gzh->gzsize != zone->elem_size) {
                                panic("Mismatched zfree or under/overflow for zone %p, recorded size: 0x%x, element size: 0x%x, address: %p\n", zone, gzh->gzsize, (uint32_t) zone->elem_size, (void *)addr);
                        }

                        char *gzc, *checkstart, *checkend;
                        if (gzalloc_uf_mode) {
                                checkstart = (char *) ((uintptr_t) gzh + sizeof(gzh));
                                checkend = (char *) ((((vm_offset_t)addr) & ~PAGE_MASK) + PAGE_SIZE);
                        } else {
                                checkstart = (char *) trunc_page_64(addr);
                                checkend = (char *)gzh;
                        }

                        for (gzc = checkstart; gzc < checkend; gzc++) {
                                if (*gzc != gzalloc_fill_pattern) {
                                        panic("GZALLOC: detected over/underflow, byte at %p, element %p, contents 0x%x from 0x%lx byte sized zone (%s) doesn't match fill pattern (%c)", gzc, addr, *gzc, zone->elem_size, zone->zone_name, gzalloc_fill_pattern);
                                }
                        }
                }

                if (!kmem_ready || gzh->gzone == GZDEADZONE) {
                        /* For now, just leak frees of early allocations
                         * performed before kmem is fully configured.
                         * They don't seem to get freed currently;
                         * consider ml_static_mfree in the future.
                         */
                        OSAddAtomic64((SInt32) (rounded_size), &gzalloc_early_free);
                        return TRUE;
                }

                if (get_preemption_level() != 0) {
                        pdzfree_count++;
                }

                if (gzfc_size) {
                        /* Either write protect or unmap the newly freed
                         * allocation
                         */
                        kr = vm_map_protect(
                                gzalloc_map,
                                saddr,
                                saddr + rounded_size + (1 * PAGE_SIZE),
                                gzalloc_prot,
                                FALSE);
                        if (kr != KERN_SUCCESS) {
                                panic("%s: vm_map_protect: %p, 0x%x", __FUNCTION__, (void *)saddr, kr);
                        }
                } else {
                        free_addr = saddr;
                }

                lock_zone(zone);
                assert(zone->zone_valid);

                /* Insert newly freed element into the protected free element
                 * cache, and rotate out the LRU element.
                 */
                if (gzfc_size) {
                        if (zone->gz.gzfc_index >= gzfc_size) {
                                zone->gz.gzfc_index = 0;
                        }
                        free_addr = zone->gz.gzfc[zone->gz.gzfc_index];
                        zone->gz.gzfc[zone->gz.gzfc_index++] = saddr;
                }

                if (free_addr) {
                        zone->count--;
                        zone->cur_size -= rounded_size;
                }

                unlock_zone(zone);

                if (free_addr) {
                        // TODO: consider using physical reads to check for
                        // corruption while on the protected freelist
                        // (i.e. physical corruption)
                        kr = vm_map_remove(
                                gzalloc_map,
                                free_addr,
                                free_addr + rounded_size + (1 * PAGE_SIZE),
                                VM_MAP_REMOVE_KUNWIRE);
                        if (kr != KERN_SUCCESS) {
                                panic("gzfree: vm_map_remove: %p, 0x%x", (void *)free_addr, kr);
                        }
                        // TODO: sysctl-ize for quick reference
                        OSAddAtomic64((SInt32)rounded_size, &gzalloc_freed);
                        OSAddAtomic64(-((SInt32) (rounded_size - zone->elem_size)), &gzalloc_wasted);
                }

                gzfreed = TRUE;
        }
        return gzfreed;
}

boolean_t
gzalloc_element_size(void *gzaddr, zone_t *z, vm_size_t *gzsz)
{
        uintptr_t a = (uintptr_t)gzaddr;
        if (__improbable(gzalloc_mode && (a >= gzalloc_map_min) && (a < gzalloc_map_max))) {
                gzhdr_t *gzh;
                boolean_t       vmef;
                vm_map_entry_t  gzvme = NULL;
                vm_map_lock_read(gzalloc_map);
                vmef = vm_map_lookup_entry(gzalloc_map, (vm_map_offset_t)a, &gzvme);
                vm_map_unlock(gzalloc_map);
                if (vmef == FALSE) {
                        panic("GZALLOC: unable to locate map entry for %p\n", (void *)a);
                }
                assertf(gzvme->vme_atomic != 0, "GZALLOC: VM map entry inconsistency, vme: %p, start: %llu end: %llu", gzvme, gzvme->vme_start, gzvme->vme_end);

                /* Locate the gzalloc metadata adjoining the element */
                if (gzalloc_uf_mode == TRUE) {
                        /* In underflow detection mode, locate the map entry describing
                         * the element, and then locate the copy of the gzalloc
                         * header at the trailing edge of the range.
                         */
                        gzh = (gzhdr_t *)(gzvme->vme_end - GZHEADER_SIZE);
                } else {
                        /* In overflow detection mode, scan forward from
                         * the base of the map entry to locate the
                         * gzalloc header.
                         */
                        uint32_t *p = (uint32_t*) gzvme->vme_start;
                        while (p < (uint32_t *) gzvme->vme_end) {
                                if (*p == GZALLOC_SIGNATURE) {
                                        break;
                                } else {
                                        p++;
                                }
                        }
                        if (p >= (uint32_t *) gzvme->vme_end) {
                                panic("GZALLOC signature missing addr %p, zone %p", gzaddr, z);
                        }
                        p++;
                        uintptr_t q = (uintptr_t) p;
                        gzh = (gzhdr_t *) (q - sizeof(gzhdr_t));
                }

                if (gzh->gzsig != GZALLOC_SIGNATURE) {
                        panic("GZALLOC signature mismatch for element %p, expected 0x%x, found 0x%x", (void *)a, GZALLOC_SIGNATURE, gzh->gzsig);
                }

                *gzsz = gzh->gzone->elem_size;
                if (__improbable((gzalloc_tracked(gzh->gzone)) == FALSE)) {
                        panic("GZALLOC: zone mismatch (%p)\n", gzh->gzone);
                }

                if (z) {
                        *z = gzh->gzone;
                }
                return TRUE;
        } else {
                return FALSE;
        }
}