From 84bf84032208e21d20ad13f3033d4fca3a512014 Mon Sep 17 00:00:00 2001 From: Olivier Dion Date: Sat, 8 Apr 2023 16:48:01 -0400 Subject: [PATCH] Fix dangling pointers in `environ'. MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 --- libguile/posix.c | 21 ++++++++++++++++++++- 1 file changed, 20 insertions(+), 1 deletion(-) diff --git a/libguile/posix.c b/libguile/posix.c index 4cf4ef383..4564cb33b 100644 --- a/libguile/posix.c +++ b/libguile/posix.c @@ -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; } }