[PATCH] libdw: check memory access in get_(u|s)leb128
Mark Wielaard
mark@klomp.org
Sat Feb 11 23:42:58 GMT 2023
Hi Aleksei,
On Wed, Jan 25, 2023 at 04:05:30PM +0000, Aleksei Vetrov via Elfutils-devel wrote:
> From: Aleksei Vetrov <vvvvvv@google.com>
>
> __libdw_get_uleb128 and __libdw_get_sleb128 should check if addrp has
> already reached the end before unrolling the first step. It is done by
> moving __libdw_max_len to the beginning of the function, which already
> has all the checks.
I did some performance tests and couldn't find any significant change.
Even with my other extra checks added in libdw and readelf.
One question about the sleb128 case though:
> static inline int64_t
> __libdw_get_sleb128 (const unsigned char **addrp, const unsigned char *end)
> {
> + const size_t max = __libdw_max_len_sleb128 (*addrp, end);
> /* Do the work in an unsigned type, but use implementation-defined
> behavior to cast to signed on return. This avoids some undefined
> behavior when shifting. */
> @@ -131,9 +133,9 @@ __libdw_get_sleb128 (const unsigned char **addrp, const unsigned char *end)
>
> /* Unroll the first step to help the compiler optimize
> for the common single-byte case. */
> - get_sleb128_step (acc, *addrp, 0);
> + if (likely (max > 0))
> + get_sleb128_step (acc, *addrp, 0);
>
> - const size_t max = __libdw_max_len_sleb128 (*addrp - 1, end);
> for (size_t i = 1; i < max; ++i)
> get_sleb128_step (acc, *addrp, i);
> if (*addrp == end)
But what about the case where *addrp > end?
After this code we will do:
/* There might be one extra byte. */
unsigned char b = **addrp;
++*addrp;
So I think we want to catch that too. Easiest imho seems to move (and
invert) the max check immediately after calculating max:
diff --git a/libdw/memory-access.h b/libdw/memory-access.h
index 1cac6af3..72348623 100644
--- a/libdw/memory-access.h
+++ b/libdw/memory-access.h
@@ -126,6 +126,9 @@ static inline int64_t
__libdw_get_sleb128 (const unsigned char **addrp, const unsigned char *end)
{
const size_t max = __libdw_max_len_sleb128 (*addrp, end);
+ if (unlikely (max == 0))
+ return INT64_MAX;
+
/* Do the work in an unsigned type, but use implementation-defined
behavior to cast to signed on return. This avoids some undefined
behavior when shifting. */
@@ -133,8 +136,7 @@ __libdw_get_sleb128 (const unsigned char **addrp, const unsigned char *end)
/* Unroll the first step to help the compiler optimize
for the common single-byte case. */
- if (likely (max > 0))
- get_sleb128_step (acc, *addrp, 0);
+ get_sleb128_step (acc, *addrp, 0);
for (size_t i = 1; i < max; ++i)
get_sleb128_step (acc, *addrp, i);
Cheers,
Mark
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