diff options
| author | Mark Wielaard <[email protected]> | 2015-09-03 10:50:58 +0200 |
|---|---|---|
| committer | Mark Wielaard <[email protected]> | 2015-09-03 10:50:58 +0200 |
| commit | b00a4fa78779ff0f304fa6cb34d49622679c86d4 (patch) | |
| tree | fb79c2b93b99634391ac54652e1df5ff60b2a674 | |
| parent | a74bdbddd5cd03f6bb633455d8c3b5f6baaf92b5 (diff) | |
readelf: handle_core_item large right shift triggers undefined behaviour.
The problem is this:
int n = ffs (w);
w >>= n;
The intent is to shift away up to (and including) the first least
significant bit in w. But w is an unsigned int, so 32 bits. And the
least significant bit could be bit 32 (ffs counts from 1). Unfortunately
a right shift equal to (or larger than) the length in bits of the left
hand operand is undefined behaviour. We expect w to be zero afterwards.
Which would terminate the while loop in the function. But since it is
undefined behaviour anything can happen. In this case, what will actually
happen is that w is unchanged, causing an infinite loop...
gcc -fsanitize=undefined will catch and warn about this when w = 0x80000000
https://bugzilla.redhat.com/show_bug.cgi?id=1259259
Signed-off-by: Mark Wielaard <[email protected]>
| -rw-r--r-- | src/ChangeLog | 4 | ||||
| -rw-r--r-- | src/readelf.c | 12 |
2 files changed, 14 insertions, 2 deletions
diff --git a/src/ChangeLog b/src/ChangeLog index 5be10750..66f7ead1 100644 --- a/src/ChangeLog +++ b/src/ChangeLog @@ -1,3 +1,7 @@ +2015-09-03 Mark Wielaard <[email protected]> + + * readelf.c (handle_core_item): Handle right shift >= 32 bits. + 2015-08-11 Mark Wielaard <[email protected]> * elflint.c (check_sections): When gnuld and a NOBITS section falls diff --git a/src/readelf.c b/src/readelf.c index d3c2b6b4..aab8b5cc 100644 --- a/src/readelf.c +++ b/src/readelf.c @@ -8474,8 +8474,16 @@ handle_core_item (Elf *core, const Ebl_Core_Item *item, const void *desc, unsigned int w = negate ? ~*i : *i; while (w != 0) { - int n = ffs (w); - w >>= n; + /* Note that a right shift equal to (or greater than) + the number of bits of w is undefined behaviour. In + particular when the least significant bit is bit 32 + (w = 0x8000000) then w >>= n is undefined. So + explicitly handle that case separately. */ + unsigned int n = ffs (w); + if (n < sizeof (w) * 8) + w >>= n; + else + w = 0; bit += n; if (lastbit != 0 && lastbit + 1 == bit) |