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_mul_overflow and get_encoded_value

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A sufficiently mad compiler optimiser can take undefined behaviour
according to the C standard as an opportunity to remove code.  Since
"data + size" might be seen to be past the end of an array,
calculating such an expression is UB.

_mul_overflow is infrastructure for later patches.

	* bucomm.h (_mul_overflow): Define.
	* dwarf.c (get_encoded_value): Avoid pointer UB.
This commit is contained in:
Alan Modra 2021-05-15 14:36:26 +09:30
parent 7c96e6120f
commit fc5e0925d4
3 changed files with 14 additions and 1 deletions
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5
binutils/ChangeLog
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@ -1,3 +1,8 @@
2021-05-15 Alan Modra <amodra@gmail.com>
* bucomm.h (_mul_overflow): Define.
* dwarf.c (get_encoded_value): Avoid pointer UB.
2021-05-13 Alan Modra <amodra@gmail.com>
PR 27861

8
binutils/bucomm.h
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@ -80,4 +80,12 @@ void *xmalloc (size_t);
void *xrealloc (void *, size_t);
#if __GNUC__ >= 7
#define _mul_overflow(a, b, res) __builtin_mul_overflow (a, b, res)
#else
/* Assumes unsigned values. Careful! Args evaluated multiple times. */
#define _mul_overflow(a, b, res) \
((*res) = (a), (*res) *= (b), (b) != 0 && (*res) / (b) != (a))
#endif
#endif /* _BUCOMM_H */

2
binutils/dwarf.c
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@ -178,7 +178,7 @@ get_encoded_value (unsigned char **pdata,
unsigned int size = size_of_encoded_value (encoding);
dwarf_vma val;
if (data + size >= end)
if (data >= end || size > (size_t) (end - data))
{
warn (_("Encoded value extends past end of section\n"));
* pdata = end;