fix crash - buffer overflow by one in fexpand#521
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The symptom, on Windows, using LESSOPEN, was that "less" would sometimes (up to 5-10%) crash with exit code 116. gdb detected heap block overflow in free(filename) in lglob. The overflow is because fexpand_copy(s, NULL) return value (and allocated size) doesn't count the final \0, but fexpand_copy(s, x) does write a final \0. This appears to be a regression of commit 1626d06, where fexpand_copy returned the strlen of the result, and fexpand allocated n+1, but commit 1626d06 changed the allocation to n. Fix it by always counting the \0 - which appears to fix the crash. fexpand_copy is only used from fexpand (once to count, and once for the actual copy), so there's no risk elsewhere from this change.
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Interestingly, it doesn't seem to crash when compiled with UCRT (either gcc or clang), but does crash when compiled with MSVCRT (either gcc or clang). Maybe the UCRT malloc leaves some space after the block, so it can avoid crashes with small overflows? Or maybe the allignment or selected block size is bigger with UCRT... EDIT: it also crashes with UCRT, just with different filename lengths. So I guess the UCRT malloc doesn't use exactly the same block sizes as MSVCRT. This PR fix the crash also with UCRT. |
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The symptom, on Windows, using LESSOPEN, was that "less" would sometimes (up to 5-10%) crash with exit code 116.
gdb detected heap block overflow in free(filename) in lglob.
The overflow is because fexpand_copy(s, NULL) return value (and allocated size) doesn't count the final \0, but fexpand_copy(s, x) does write a final \0.
This appears to be a regression of commit 1626d06, where fexpand_copy returned the strlen of the result, and fexpand allocated n+1, but commit 1626d06 changed the allocation to n.
Fix it by always counting the \0 - which appears to fix the crash.
fexpand_copy is only used from fexpand (once to count, and once for the actual copy), so there's no risk elsewhere from this change.