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Fix dangling pointers in `environ'.

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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.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>
This commit is contained in:
Olivier Dion 2023-04-08 16:48:01 -04:00 • committed by Ludovic Courtès
parent c7d170c5d1
commit 84bf840322
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1 changed files with 20 additions and 1 deletions
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21
libguile/posix.c
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@ -1755,11 +1755,30 @@ SCM_DEFINE (scm_environ, "environ", 0, 1, 0,
"then the return value is unspecified.")
#define FUNC_NAME s_scm_environ
{
/* Accessing `environ' directly in a multi-threaded program is
undefined behavior since at anytime it could point to anything else
while reading it. Not only that, but all accesses are protected by
an internal mutex of libc. Thus, it is only truly safe to modify
the environment directly in a single-threaded program. */
if (scm_ilength (scm_all_threads ()) != 1)
scm_display
(scm_from_latin1_string
("warning: call to environ while multiple threads are running;\n"
" further behavior unspecified.\n"),
scm_current_warning_port ());
if (SCM_UNBNDP (env))
return scm_makfromstrs (-1, environ);
else
{
environ = scm_i_allocate_string_pointers (env);
/* Arrange to not use GC-allocated storage for what goes into
'environ' as libc might reallocate it behind our back. */
clearenv ();
while (!scm_is_null (env))
{
scm_putenv (scm_car (env));
env = scm_cdr (env);
}
return SCM_UNSPECIFIED;
}
}