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[PATCH] slab: hexdump for check_poison
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From: Manfred Spraul <[email protected]>

The patch is designed improve the diagnostics which are presented when the
slab memory poison detector triggers.


check_poison_obj checks for write accesses after kfree by comparing the
object contents with the poison value.  The current implementation contains
several flaws:

- it accepts both POISON_BEFORE and POISON_AFTER.  check_poison_obj is
  only called with POISON_AFTER poison bytes.  Fix: only accept
  POISON_AFTER.

- the output is unreadable.  Fix: use hexdump.

- if a large objects is corrupted, then the relevant lines can scroll of
  the screen/dmesg buffer.  Fix: line limit.

- it can access addresses behind the end of the object, which can oops
  with CONFIG_DEBUG_PAGEALLOC.  Fix: bounds checks.

Additionally, the patch contains the following changes:

- rename POISON_BEFORE and POISON_AFTER to POISON_FREE and POISON_INUSE.
  The old names are ambiguous.

- use the new hexdump object function in ptrinfo.

- store_stackinfo was called with wrong parameters: it should store
  caller, i.e.  __builtin_return_address(0), not POISON_AFTER in the
  object.

- dump both the object before and after the corrupted one, not just the
  one after.

Example output:
<<<
Slab corruption: start=194e708c, len=2048
Redzone: 0x5a2cf071/0x5a2cf071.
Last user: [<02399d7c>](dummy_init_module+0x1c/0xb0)
010: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 7b
030: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 63
Prev obj: start=194e6880, len=2048
Redzone: 0x5a2cf071/0x5a2cf071.
Last user: [<00000000>](0x0)
000: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b
010: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b
<<<
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Andrew Morton authored and David S. Miller committed Feb 19, 2004
1 parent c03544d commit 86c662d
Showing 1 changed file with 97 additions and 59 deletions.
156 changes: 97 additions & 59 deletions mm/slab.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -357,8 +357,8 @@ struct kmem_cache_s {
#define RED_ACTIVE 0x170FC2A5UL /* when obj is active */

/* ...and for poisoning */
#define POISON_BEFORE 0x5a /* for use-uninitialised poisoning */
#define POISON_AFTER 0x6b /* for use-after-free poisoning */
#define POISON_INUSE 0x5a /* for use-uninitialised poisoning */
#define POISON_FREE 0x6b /* for use-after-free poisoning */
#define POISON_END 0xa5 /* end-byte of poisoning */

/* memory layout of objects:
Expand Down Expand Up @@ -887,60 +887,105 @@ static void poison_obj(kmem_cache_t *cachep, void *addr, unsigned char val)
*(unsigned char *)(addr+size-1) = POISON_END;
}

static void *scan_poisoned_obj(unsigned char* addr, unsigned int size)
static void dump_line(char *data, int offset, int limit)
{
unsigned char *end;

end = addr + size - 1;
int i;
printk(KERN_ERR "%03x:", offset);
for (i=0;i<limit;i++) {
printk(" %02x", (unsigned char)data[offset+i]);
}
printk("\n");
}
#endif

static void print_objinfo(kmem_cache_t *cachep, void *objp, int lines)
{
#if DEBUG
int i, size;
char *realobj;

for (; addr < end; addr++) {
if (*addr != POISON_BEFORE && *addr != POISON_AFTER)
return addr;
if (cachep->flags & SLAB_RED_ZONE) {
printk(KERN_ERR "Redzone: 0x%lx/0x%lx.\n",
*dbg_redzone1(cachep, objp),
*dbg_redzone2(cachep, objp));
}
if (*addr != POISON_END)
return addr;
return NULL;

if (cachep->flags & SLAB_STORE_USER) {
printk(KERN_ERR "Last user: [<%p>]", *dbg_userword(cachep, objp));
print_symbol("(%s)", (unsigned long)*dbg_userword(cachep, objp));
printk("\n");
}
realobj = (char*)objp+obj_dbghead(cachep);
size = cachep->objsize;
for (i=0; i<size && lines;i+=16, lines--) {
int limit;
limit = 16;
if (i+limit > size)
limit = size-i;
dump_line(realobj, i, limit);
}
#endif
}

#if DEBUG

static void check_poison_obj(kmem_cache_t *cachep, void *objp)
{
void *end;
void *realobj;
int size = obj_reallen(cachep);

realobj = objp+obj_dbghead(cachep);

end = scan_poisoned_obj(realobj, size);
if (end) {
int s;
printk(KERN_ERR "Slab corruption: start=%p, expend=%p, "
"problemat=%p\n", realobj, realobj+size-1, end);
if (cachep->flags & SLAB_STORE_USER) {
printk(KERN_ERR "Last user: [<%p>]", *dbg_userword(cachep, objp));
print_symbol("(%s)", (unsigned long)*dbg_userword(cachep, objp));
printk("\n");
char *realobj;
int size, i;
int lines = 0;

realobj = (char*)objp+obj_dbghead(cachep);
size = obj_reallen(cachep);

for (i=0;i<size;i++) {
char exp = POISON_FREE;
if (i == size-1)
exp = POISON_END;
if (realobj[i] != exp) {
int limit;
/* Mismatch ! */
/* Print header */
if (lines == 0) {
printk(KERN_ERR "Slab corruption: start=%p, len=%d\n",
realobj, size);
print_objinfo(cachep, objp, 0);
}
/* Hexdump the affected line */
i = (i/16)*16;
limit = 16;
if (i+limit > size)
limit = size-i;
dump_line(realobj, i, limit);
i += 16;
lines++;
/* Limit to 5 lines */
if (lines > 5)
break;
}
printk(KERN_ERR "Data: ");
for (s = 0; s < size; s++) {
if (((char*)realobj)[s] == POISON_BEFORE)
printk(".");
else if (((char*)realobj)[s] == POISON_AFTER)
printk("*");
else
printk("%02X ", ((unsigned char*)realobj)[s]);
}
if (lines != 0) {
/* Print some data about the neighboring objects, if they
* exist:
*/
struct slab *slabp = GET_PAGE_SLAB(virt_to_page(objp));
int objnr;

objnr = (objp-slabp->s_mem)/cachep->objsize;
if (objnr) {
objp = slabp->s_mem+(objnr-1)*cachep->objsize;
realobj = (char*)objp+obj_dbghead(cachep);
printk(KERN_ERR "Prev obj: start=%p, len=%d\n",
realobj, size);
print_objinfo(cachep, objp, 2);
}
printk("\n");
printk(KERN_ERR "Next: ");
for (; s < size + 32; s++) {
if (((char*)realobj)[s] == POISON_BEFORE)
printk(".");
else if (((char*)realobj)[s] == POISON_AFTER)
printk("*");
else
printk("%02X ", ((unsigned char*)realobj)[s]);
if (objnr+1 < cachep->num) {
objp = slabp->s_mem+(objnr+1)*cachep->objsize;
realobj = (char*)objp+obj_dbghead(cachep);
printk(KERN_ERR "Next obj: start=%p, len=%d\n",
realobj, size);
print_objinfo(cachep, objp, 2);
}
printk("\n");
slab_error(cachep, "object was modified after freeing");
}
}
#endif
Expand Down Expand Up @@ -1495,7 +1540,7 @@ static void cache_init_objs (kmem_cache_t * cachep,
#if DEBUG
/* need to poison the objs? */
if (cachep->flags & SLAB_POISON)
poison_obj(cachep, objp, POISON_BEFORE);
poison_obj(cachep, objp, POISON_FREE);
if (cachep->flags & SLAB_STORE_USER)
*dbg_userword(cachep, objp) = NULL;

Expand Down Expand Up @@ -1714,13 +1759,13 @@ static inline void *cache_free_debugcheck (kmem_cache_t * cachep, void * objp, v
if (cachep->flags & SLAB_POISON) {
#ifdef CONFIG_DEBUG_PAGEALLOC
if ((cachep->objsize % PAGE_SIZE) == 0 && OFF_SLAB(cachep)) {
store_stackinfo(cachep, objp, POISON_AFTER);
store_stackinfo(cachep, objp, (unsigned long)caller);
kernel_map_pages(virt_to_page(objp), cachep->objsize/PAGE_SIZE, 0);
} else {
poison_obj(cachep, objp, POISON_AFTER);
poison_obj(cachep, objp, POISON_FREE);
}
#else
poison_obj(cachep, objp, POISON_AFTER);
poison_obj(cachep, objp, POISON_FREE);
#endif
}
#endif
Expand Down Expand Up @@ -1877,7 +1922,7 @@ cache_alloc_debugcheck_after(kmem_cache_t *cachep,
#else
check_poison_obj(cachep, objp);
#endif
poison_obj(cachep, objp, POISON_BEFORE);
poison_obj(cachep, objp, POISON_INUSE);
}
if (cachep->flags & SLAB_STORE_USER)
*dbg_userword(cachep, objp) = caller;
Expand Down Expand Up @@ -2868,14 +2913,7 @@ void ptrinfo(unsigned long addr)
kernel_map_pages(virt_to_page(objp),
c->objsize/PAGE_SIZE, 1);

if (c->flags & SLAB_RED_ZONE)
printk("redzone: 0x%lx/0x%lx.\n",
*dbg_redzone1(c, objp),
*dbg_redzone2(c, objp));

if (c->flags & SLAB_STORE_USER)
printk("Last user: %p.\n",
*dbg_userword(c, objp));
print_objinfo(c, objp, 2);
}
spin_unlock_irqrestore(&c->spinlock, flags);

Expand Down

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