[Bug libc/20847] tst-vfork3 failure

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https://sourceware.org/bugzilla/show_bug.cgi?id=20847

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https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=6c9e1be87a37bfac0bf6c80a38171383ac3527e6

commit 6c9e1be87a37bfac0bf6c80a38171383ac3527e6
Author: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Date:   Mon Nov 21 11:06:15 2016 -0200

    Fix writes past the allocated array bounds in execvpe (BZ#20847)

    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

-----------------------------------------------------------------------

Summary of changes:
 ChangeLog       |    7 +++++++
 posix/execvpe.c |   15 ++++++++++-----
 2 files changed, 17 insertions(+), 5 deletions(-)

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