This patch fixes an invalid write out or stack allocated buffer in
2 places at execvpe implementation:
1. On 'maybe_script_execute' function where it allocates the new
argument list and it does not account that a minimum of argc
plus 3 elements (default shell path, script name, arguments,
and ending null pointer) should be considered. The straightforward
fix is just to take account of the correct list size on argument
copy.
2. On '__execvpe' where the executable file name lenght may not
account for ending '\0' and thus subsequent path creation may
write past array bounds because it requires to add the terminating
null. The fix is to change how to calculate the executable name
size to add the final '\0' and adjust the rest of the code
accordingly.
As described in GCC bug report 78433 [1], these issues were masked off by
GCC because it allocated several bytes more than necessary so that many
off-by-one bugs went unnoticed.
Checked on x86_64 with a latest GCC (7.0.0 20161121) with -O3 on CFLAGS.
[BZ #20847]
* posix/execvpe.c (maybe_script_execute): Remove write past allocated
array bounds.
(__execvpe): Likewise.
[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78433
