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SubjectMemory modified after freeing in 2.5.63?
Hi,
for some time I'm using patch attached at the end of this email,
which modifies check_poison function to not only verify that
last byte is POISON_END, but also that all preceeding bytes are
either POISON_BEFORE or POISON_AFTER bytes.

And now when I returned from my month vacation and upgraded
from 2.5.52 to 2.5.63, when dselect/apt updates dozens of packages,
I'm getting memory corruption reports as shown below - 22nd byte
in vm_area_struct - which looks like that VM_ACCOUNT in vm_flags
is set after vma is freeed... Any clue? Setting VM_ACCOUNT
in mremap.c:move_vma after calling do_unmap() looks suspicious
to me, but as I know almost nothing about MM...

Kernel is SMP, 2.5.63, with all debugging except stack frame
enabled, running on UP P4.
Thanks,
Petr Vandrovec
vandrove@vc.cvut.cz

(* = POISON_AFTER, . = POISON_BEFORE, number = other value)

Corruption! start=d134abb4, expend=d134abf3, problemat=d134abca
Data: **********************7B ****************************************A5
Next: 71 F0 2C .A5 C2 0F 17 20 E2 32 DA 00 40 01 40 00 50 01 40 A4 9F 9A D9 25 00 00 00 73 00 10 00
slab error in check_poison_obj(): cache `vm_area_struct': object was modified after freeing
Call Trace:
[<c014165c>] check_poison_obj+0xf7/0x130
[<c0143ba9>] cache_alloc_debugcheck_after+0xbd/0xc5
[<c0142ac7>] kmem_cache_alloc+0x6c/0xaa
[<c014cb50>] split_vma+0x56/0x123
[<c014ccdd>] do_munmap+0xc0/0x1cd
[<c014b832>] sys_brk+0x127/0x12b
[<c01096cf>] syscall_call+0x7/0xb

Corruption! start=d134a734, expend=d134a773, problemat=d134a74a
Data: **********************7B ****************************************A5
Next: 71 F0 2C .71 F0 2C .************************
slab error in check_poison_obj(): cache `vm_area_struct': object was modified after freeing
Call Trace:
[<c014165c>] check_poison_obj+0xf7/0x130
[<c0143ba9>] cache_alloc_debugcheck_after+0xbd/0xc5
[<c0142212>] kmem_flagcheck+0x1f/0x2b
[<c0142ac7>] kmem_cache_alloc+0x6c/0xaa
[<c014e27f>] move_vma+0x269/0x598
[<c012b0be>] update_wall_time+0xd/0x36
[<c014d3f8>] arch_get_unmapped_area+0x72/0xb7
[<c014e83b>] do_mremap+0x28d/0x3eb
[<c014ea0a>] sys_mremap+0x71/0x9a
[<c01096cf>] syscall_call+0x7/0xb

Corruption! start=d13b76a4, expend=d13b76e3, problemat=d13b76ba
Data: **********************7B ****************************************A5
Next: 71 F0 2C .A5 C2 0F 17 90 A8 F3 D9 00 20 20 40 00 20 21 40 94 01 B6 D9 25 00 00 00 75 00 00 00
slab error in check_poison_obj(): cache `vm_area_struct': object was modified after freeing
Call Trace:
[<c014165c>] check_poison_obj+0xf7/0x130
[<c0143ba9>] cache_alloc_debugcheck_after+0xbd/0xc5
[<c0142ac7>] kmem_cache_alloc+0x6c/0xaa
[<c014cb50>] split_vma+0x56/0x123
[<c014ccdd>] do_munmap+0xc0/0x1cd
[<c014b832>] sys_brk+0x127/0x12b
[<c01096cf>] syscall_call+0x7/0xb

Corruption! start=d134aecc, expend=d134af0b, problemat=d134aee2
Data: **********************7B ****************************************A5
Next: 71 F0 2C .71 F0 2C .************************
slab error in check_poison_obj(): cache `vm_area_struct': object was modified after freeing
Call Trace:
[<c014165c>] check_poison_obj+0xf7/0x130
[<c0143ba9>] cache_alloc_debugcheck_after+0xbd/0xc5
[<c0142ac7>] kmem_cache_alloc+0x6c/0xaa
[<c014e27f>] move_vma+0x269/0x598
[<c012b0be>] update_wall_time+0xd/0x36
[<c014d3f8>] arch_get_unmapped_area+0x72/0xb7
[<c014e83b>] do_mremap+0x28d/0x3eb
[<c014ea0a>] sys_mremap+0x71/0x9a
[<c01096cf>] syscall_call+0x7/0xb

diff -urdN linux/mm/slab.c linux/mm/slab.c
--- linux/mm/slab.c 2003-02-24 20:20:02.000000000 +0000
+++ linux/mm/slab.c 2003-02-24 20:54:51.000000000 +0000
@@ -768,6 +768,20 @@
*(unsigned char *)(addr+size-1) = POISON_END;
}

+static void* fprob(unsigned char* addr, unsigned int size) {
+ unsigned char* end;
+
+ end = addr + size - 1;
+
+ for (; addr < end; addr++) {
+ if (*addr != POISON_BEFORE && *addr != POISON_AFTER)
+ return addr;
+ }
+ if (*addr != POISON_END)
+ return addr;
+ return NULL;
+}
+
static void check_poison_obj(kmem_cache_t *cachep, void *addr)
{
int size = cachep->objsize;
@@ -776,9 +790,32 @@
addr += BYTES_PER_WORD;
size -= 2*BYTES_PER_WORD;
}
- end = memchr(addr, POISON_END, size);
- if (end != (addr+size-1))
+ end = fprob(addr, size);
+ if (end) {
+ int s;
+ printk(KERN_ERR "Corruption! start=%p, expend=%p, problemat=%p\n", addr, addr+size-1, end);
+ printk(KERN_ERR "Data: ");
+ for (s = 0; s < size; s++) {
+ if (((char*)addr)[s] == POISON_BEFORE)
+ printk(".");
+ else if (((char*)addr)[s] == POISON_AFTER)
+ printk("*");
+ else
+ printk("%02X ", ((unsigned char*)addr)[s]);
+ }
+ printk("\n");
+ printk(KERN_ERR "Next: ");
+ for (; s < size + 32; s++) {
+ if (((char*)addr)[s] == POISON_BEFORE)
+ printk(".");
+ else if (((char*)addr)[s] == POISON_AFTER)
+ printk("*");
+ else
+ printk("%02X ", ((unsigned char*)addr)[s]);
+ }
+ printk("\n");
slab_error(cachep, "object was modified after freeing");
+ }
}
#endif

-
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