From: "Ludovic Courtès" <ludo@gnu.org>
To: Olivier Dion <olivier.dion@polymtl.ca>
Cc: 62729-done@debbugs.gnu.org, Olivier Dion <olivier-dion@proton.me>
Subject: bug#62729: [PATCH] Fix dangling pointers in `environ'
Date: Sun, 16 Jul 2023 22:18:54 +0200 [thread overview]
Message-ID: <87sf9nr4nl.fsf@gnu.org> (raw)
In-Reply-To: <20230408204801.10408-1-olivier.dion@polymtl.ca> (Olivier Dion's message of "Sat, 8 Apr 2023 16:48:01 -0400")
Hi Olivier,
Olivier Dion <olivier.dion@polymtl.ca> skribis:
> From: Olivier Dion <olivier-dion@proton.me>
>
> When calling `environ', Guile set the global variable `environ' to a
> list allocated with the GC. Strings in it are also allocated with the
> GC.
>
> However, if an user call the Scheme setenv() procedure, the resulting
> call to putenv() in libc might reallocate `environ' to a new pointer
> while copying sub-pointers owned by Guile in it.
>
> This results in the GC marking these strings for reclamation when they
> are actually still present in `environ'. Thus, the values in the
> environment are now undefined.
>
> To fix this, Guile should only manipulate the `environ' using the
> standard libc functions. This ensures that concurrent modification of
> it is safe in multi-threaded program. Therefore, the procedure
> `environ' now call the libc clearenv() procedure to purge the
> environment. Then, the desired values are put in `environ' using
> scm_putenv(). At the end, no GC allocated memory is put in `environ'.
>
> Also, since `environ' can be changed at anytime in a multi-thread
> program, emit a warning stipulating that the result is undefined
> behavior if multiple threads are created in the program. Consider for
> example a thread iterating over `environ' while another one do a call to
> putenv(). The latter would do a realloc() on `environ' and thus the old
> array read by the former now contains garbage.
>
> On system where clearenv() is not present, an atomic store of NULL with
> sequential consistency to `environ' should be sufficient but see the
> NOTES of clearenv(3).
>
> * libguile/posix.c (scm_environ): Do not store GC allocated memory in
> environ.
Thanks for the clear explanation and patch. Finally applied with an
added comment in the code.
Ludo’.
prev parent reply other threads:[~2023-07-16 20:18 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-04-08 20:48 bug#62729: [PATCH] Fix dangling pointers in `environ' Olivier Dion via Bug reports for GUILE, GNU's Ubiquitous Extension Language
2023-07-16 20:18 ` Ludovic Courtès [this message]
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