Fix: libc wrapper: use initial-exec for malloc_nesting TLS

Use the initial-exec TLS model for the malloc_nesting nesting guard
variable to ensure that the glibc implementation of the TLS access don't
trigger infinite recursion by calling the memory allocator wrapper
functions, which can happen with global-dynamic.

Considering that the libc wrapper is meant to be loaded with LD_PRELOAD
anyway (never with dlopen(3)), we always expect the libc to have enough
space to hold the malloc_nesting variable.

In addition to change the malloc_nesting from global-dynamic to
initial-exec, this removes the URCU TLS compatibility layer from the
libc wrapper, which is a good thing: this compatibility layer relies
on pthread key and calloc internally, which makes it a bad fit for TLS
accesses guarding access to malloc wrappers, due to possible infinite
recursion.

Link: https://lists.lttng.org/pipermail/lttng-dev/2024-January/030697.html
Reported-by: Florian Weimer <fweimer@redhat.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I72c42bc09c1a06e2922b184b85abeb9c94200ee2

diff --git a/liblttng-ust-libc-wrapper/lttng-ust-malloc.c b/liblttng-ust-libc-wrapper/lttng-ust-malloc.c
index 9e7d1e63f2893668343adb172406497b76f81ca6..84641ddaebcf2d2dfbe5a6e9d1d39e954f047179 100644 (file)
--- a/liblttng-ust-libc-wrapper/lttng-ust-malloc.c
+++ b/liblttng-ust-libc-wrapper/lttng-ust-malloc.c
@@ -30,7 +30,6 @@
 #include <urcu/system.h>
 #include <urcu/uatomic.h>
 #include <urcu/compiler.h>
-#include <urcu/tls-compat.h>
 #include <urcu/arch.h>
 #include <lttng/align.h>
 #include <helper.h>
@@ -92,13 +91,13 @@ void ust_malloc_spin_unlock(pthread_mutex_t *lock)
        uatomic_set(&ust_malloc_lock, 0);
 }
 
-#define calloc static_calloc
-#define pthread_mutex_lock ust_malloc_spin_lock
-#define pthread_mutex_unlock ust_malloc_spin_unlock
-static DEFINE_URCU_TLS(int, malloc_nesting);
-#undef pthread_mutex_unlock
-#undef pthread_mutex_lock
-#undef calloc
+/*
+ * Use initial-exec TLS model for the malloc_nesting nesting guard
+ * variable to ensure that the glibc implementation of the TLS access
+ * don't trigger infinite recursion by calling the memory allocator
+ * wrapper functions, which could happen with global-dynamic.
+ */
+static __thread __attribute__((tls_model("initial-exec"))) int malloc_nesting;
 
 /*
  * Static allocator to use when initially executing dlsym(). It keeps a
@@ -257,7 +256,7 @@ void *malloc(size_t size)
 {
        void *retval;
 
-       URCU_TLS(malloc_nesting)++;
+       malloc_nesting++;
        if (cur_alloc.malloc == NULL) {
                lookup_all_symbols();
                if (cur_alloc.malloc == NULL) {
@@ -266,17 +265,17 @@ void *malloc(size_t size)
                }
        }
        retval = cur_alloc.malloc(size);
-       if (URCU_TLS(malloc_nesting) == 1) {
+       if (malloc_nesting == 1) {
                tracepoint(lttng_ust_libc, malloc,
                        size, retval, LTTNG_UST_CALLER_IP());
        }
-       URCU_TLS(malloc_nesting)--;
+       malloc_nesting--;
        return retval;
 }
 
 void free(void *ptr)
 {
-       URCU_TLS(malloc_nesting)++;
+       malloc_nesting++;
        /*
         * Check whether the memory was allocated with
         * static_calloc_align, in which case there is nothing to free.
@@ -286,7 +285,7 @@ void free(void *ptr)
                goto end;
        }
 
-       if (URCU_TLS(malloc_nesting) == 1) {
+       if (malloc_nesting == 1) {
                tracepoint(lttng_ust_libc, free,
                        ptr, LTTNG_UST_CALLER_IP());
        }
@@ -300,14 +299,14 @@ void free(void *ptr)
        }
        cur_alloc.free(ptr);
 end:
-       URCU_TLS(malloc_nesting)--;
+       malloc_nesting--;
 }
 
 void *calloc(size_t nmemb, size_t size)
 {
        void *retval;
 
-       URCU_TLS(malloc_nesting)++;
+       malloc_nesting++;
        if (cur_alloc.calloc == NULL) {
                lookup_all_symbols();
                if (cur_alloc.calloc == NULL) {
@@ -316,11 +315,11 @@ void *calloc(size_t nmemb, size_t size)
                }
        }
        retval = cur_alloc.calloc(nmemb, size);
-       if (URCU_TLS(malloc_nesting) == 1) {
+       if (malloc_nesting == 1) {
                tracepoint(lttng_ust_libc, calloc,
                        nmemb, size, retval, LTTNG_UST_CALLER_IP());
        }
-       URCU_TLS(malloc_nesting)--;
+       malloc_nesting--;
        return retval;
 }
 
@@ -328,7 +327,7 @@ void *realloc(void *ptr, size_t size)
 {
        void *retval;
 
-       URCU_TLS(malloc_nesting)++;
+       malloc_nesting++;
        /*
         * Check whether the memory was allocated with
         * static_calloc_align, in which case there is nothing
@@ -369,11 +368,11 @@ void *realloc(void *ptr, size_t size)
        }
        retval = cur_alloc.realloc(ptr, size);
 end:
-       if (URCU_TLS(malloc_nesting) == 1) {
+       if (malloc_nesting == 1) {
                tracepoint(lttng_ust_libc, realloc,
                        ptr, size, retval, LTTNG_UST_CALLER_IP());
        }
-       URCU_TLS(malloc_nesting)--;
+       malloc_nesting--;
        return retval;
 }
 
@@ -381,7 +380,7 @@ void *memalign(size_t alignment, size_t size)
 {
        void *retval;
 
-       URCU_TLS(malloc_nesting)++;
+       malloc_nesting++;
        if (cur_alloc.memalign == NULL) {
                lookup_all_symbols();
                if (cur_alloc.memalign == NULL) {
@@ -390,12 +389,12 @@ void *memalign(size_t alignment, size_t size)
                }
        }
        retval = cur_alloc.memalign(alignment, size);
-       if (URCU_TLS(malloc_nesting) == 1) {
+       if (malloc_nesting == 1) {
                tracepoint(lttng_ust_libc, memalign,
                        alignment, size, retval,
                        LTTNG_UST_CALLER_IP());
        }
-       URCU_TLS(malloc_nesting)--;
+       malloc_nesting--;
        return retval;
 }
 
@@ -403,7 +402,7 @@ int posix_memalign(void **memptr, size_t alignment, size_t size)
 {
        int retval;
 
-       URCU_TLS(malloc_nesting)++;
+       malloc_nesting++;
        if (cur_alloc.posix_memalign == NULL) {
                lookup_all_symbols();
                if (cur_alloc.posix_memalign == NULL) {
@@ -412,21 +411,15 @@ int posix_memalign(void **memptr, size_t alignment, size_t size)
                }
        }
        retval = cur_alloc.posix_memalign(memptr, alignment, size);
-       if (URCU_TLS(malloc_nesting) == 1) {
+       if (malloc_nesting == 1) {
                tracepoint(lttng_ust_libc, posix_memalign,
                        *memptr, alignment, size,
                        retval, LTTNG_UST_CALLER_IP());
        }
-       URCU_TLS(malloc_nesting)--;
+       malloc_nesting--;
        return retval;
 }
 
-static
-void lttng_ust_fixup_malloc_nesting_tls(void)
-{
-       asm volatile ("" : : "m" (URCU_TLS(malloc_nesting)));
-}
-
 __attribute__((constructor))
 void lttng_ust_malloc_wrapper_init(void)
 {
@@ -434,7 +427,6 @@ void lttng_ust_malloc_wrapper_init(void)
        if (cur_alloc.calloc) {
                return;
        }
-       lttng_ust_fixup_malloc_nesting_tls();
        /*
         * Ensure the allocator is in place before the process becomes
         * multithreaded.