[PATCH] stdlib: Support malloc-managed environ arrays for compatibility

[H.J. Lu]

On Wed, Jan 22, 2025 at 8:49 PM Florian Weimer <fweimer@redhat.com> wrote:
>
> Some allocations set environ to a heap-allocated pointer, call
> setenv (expecting it to call realloc), free environ, and then
> restore the original environ pointer.  This breaks after
> commit 7a61e7f557a97ab597d6fca5e2d1f13f65685c61 ("stdlib: Make
> getenv thread-safe in more cases") because after the setenv call,
> the environ pointer does not point to the start of a heap allocation.
> Instead, setenv creates a separate allocation and changes environ
> to point into that.  This means that the free call in the application
> results in heap corruption.
>
> The interim approach was more compatible with other libcs because
> it does not assume that the incoming environ pointer is allocated
> as if by malloc (if it was written by the application).  However,
> it seems to be more important to stay compatible with previous
> glibc version: assume the incoming pointer is heap allocated,
> and preserve this property after setenv calls.
>
> Tested on x86_64-linux-gnu.
>
> I'd prefer to get this into the upcoming
> release if possible, assuming that I can get a review in a timely
> fashion.
>
> Thanks,
> Florian
>
> ---
>  csu/init-first.c           |  1 +
>  csu/libc-start.c           |  1 +
>  include/unistd.h           |  3 +++
>  posix/environ.c            |  2 ++
>  stdlib/Makefile            |  1 +
>  stdlib/setenv.c            | 64 +++++++++++++++++++++++-----------------------
>  stdlib/tst-setenv-malloc.c | 64 ++++++++++++++++++++++++++++++++++++++++++++++
>  7 files changed, 104 insertions(+), 32 deletions(-)
>

On x32, I got

[hjl@gnu-tgl-3 glibc-x32-cet]$ env -i
LOCPATH=/export/build/gnu/tools-build/glibc-x32-cet/build-x86_64-linux/localedata/
LC_CTYPE=de_DE.UTF-8
/export/build/gnu/tools-build/glibc-x32-cet/build-x86_64-linux/localedata/bug-setlocale1
realloc(): invalid pointer
Aborted (core dumped)
[hjl@gnu-tgl-3 glibc-x32-cet]$