Created attachment 54786 [details] Preprocessed reproducer Link: <https://inbox.sourceware.org/gcc/45c0584d-b326-a975-7ebc-cef76e154530@gmail.com/T/#u> With both GCC 12.2.0 (Debian), and GCC 13.0.1 20230315 (built from source), I can reproduce these false positives. I'm on Debian Sid with libbsd-dev 0.11.7-4, and libc-dev 2.36-8. The reproducer program is a small program that checks a password against a hardcoded string, and conditionally prints "validated". I wrote it precisely to demonstrate how [[gnu::malloc(deallocator)]] can be used to ensure that passwords are not leaked in memory, but I found out that it fails to detect some conditions. Here's the program (it uses agetpass(), as defined in the shadow project): $ cat pass.c #include <err.h> #include <errno.h> #include <limits.h> #include <readpassphrase.h> #include <stdlib.h> #include <string.h> #include <unistd.h> #define PASS_MAX BUFSIZ - 1 #define MALLOCARRAY(n, type) ((type *) mallocarray(n, sizeof(type))) [[gnu::malloc, gnu::malloc(free)]] void *mallocarray(size_t nmemb, size_t size); void erase_pass(char *pass); [[gnu::malloc(erase_pass)]] char *agetpass(const char *prompt); void do_work(void) { char *pass; pass = agetpass("Enter your password: "); if (pass == NULL) err(EXIT_FAILURE, "agetpass"); if (strcmp(pass, "secret") == 0) puts("validated"); /* erase_pass() zeroes the memory (think of memset(3), or bzero(3)) and then releases the memory to the system (think of free(3)). If you only call free(pass), then you release the memory to the system without zeroing it. Remember it contains a password! We would be leaking a password into the system memory, which can later be assigned to a different process. So, we should call erase_pass() as soon as possible, but let's say we forgot, and just call free(): */ #if defined(BUG_1) // We forgot to zero the memory. free(pass); // GCC correctly catches this as -Wmismatched-dealloc #elif defined(BUG_2) // We zeroed, but forgot to free(3). bzero(pass, PASS_MAX + 2); // GCC misses this. #elif defined(BUG_3) // We forgot both of them. // GCC also misses this. #else erase_pass(pass); // OK, but 2 false positives. #endif } int main(void) { do_work(); for (;;) sleep(1); } void * mallocarray(size_t nmemb, size_t size) { return reallocarray(NULL, nmemb, size); } char * agetpass(const char *prompt) { char *pass; size_t len; pass = MALLOCARRAY(PASS_MAX + 2, char); if (pass == NULL) return NULL; if (readpassphrase(prompt, pass, PASS_MAX + 2, RPP_REQUIRE_TTY) == NULL) goto fail; len = strlen(pass); if (len == PASS_MAX + 1) { errno = ENOBUFS; goto fail; } return pass; fail: freezero(pass, PASS_MAX + 2); return NULL; } void erase_pass(char *pass) { freezero(pass, PASS_MAX + 2); } This shows the false positives: $ cc -Wall -Wextra pass.c $(pkgconf --cflags --libs libbsd-overlay) -fanalyzer -O3 pass.c: In function ‘agetpass’: pass.c:84:12: warning: leak of ‘pass’ [CWE-401] [-Wanalyzer-malloc-leak] 84 | if (pass == NULL) | ^ ‘do_work’: events 1-3 | | 22 | do_work(void) | | ^~~~~~~ | | | | | (1) entry to ‘do_work’ |...... | 26 | pass = agetpass("Enter your password: "); | | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | | | | | (2) allocated here | | (3) calling ‘agetpass’ from ‘do_work’ | +--> ‘agetpass’: events 4-5 | | 78 | agetpass(const char *prompt) | | ^~~~~~~~ | | | | | (4) entry to ‘agetpass’ |...... | 84 | if (pass == NULL) | | ~ | | | | | (5) ‘pass’ leaks here; was allocated at (2) | pass.c:91:12: warning: leak of ‘pass’ [CWE-401] [-Wanalyzer-malloc-leak] 91 | if (len == PASS_MAX + 1) { | ^ ‘do_work’: events 1-3 | | 22 | do_work(void) | | ^~~~~~~ | | | | | (1) entry to ‘do_work’ |...... | 26 | pass = agetpass("Enter your password: "); | | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | | | | | (2) allocated here | | (3) calling ‘agetpass’ from ‘do_work’ | +--> ‘agetpass’: events 4-9 | | 78 | agetpass(const char *prompt) | | ^~~~~~~~ | | | | | (4) entry to ‘agetpass’ |...... | 84 | if (pass == NULL) | | ~ | | | | | (5) following ‘false’ branch... |...... | 87 | if (readpassphrase(prompt, pass, PASS_MAX + 2, RPP_REQUIRE_TTY) == NULL) | | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | | || | | |(6) ...to here | | (7) following ‘false’ branch... |...... | 90 | len = strlen(pass); | | ~~~~~~~~~~~~ | | | | | (8) ...to here | 91 | if (len == PASS_MAX + 1) { | | ~ | | | | | (9) following ‘false’ branch (when ‘len != 8192’)... | ‘agetpass’: event 10 | |cc1: | (10): ...to here | ‘agetpass’: event 11 | | 91 | if (len == PASS_MAX + 1) { | | ^ | | | | | (11) ‘pass’ leaks here; was allocated at (2) | Maybe I'm missing something, but I don't think falanyzer is correct here. For the false negatives you just need to compile the above with -DBUG_2 or -DBUG_3. I didn't copy here the results, because well, it's a negative, so it simply shows the same as the above (so the false negatives and false positives happen in the same compilation, which is quite confusing).
I can still reproduce it with GCC-13 and glibc 2.36: $ cat glibc.c #include <stdio.h> int main(void) { printf("glibc version: %d.%d\n", __GLIBC__, __GLIBC_MINOR__); } $ gcc-13 -Wall -Wextra glibc.c $ ./a.out glibc version: 2.36 $ gcc-13 --version gcc-13 (Debian 13.1.0-1) 13.1.0 Copyright (C) 2023 Free Software Foundation, Inc. This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
This is probably because there's no way to mark a function as being a valid deallocator (i.e., the converse of [[gnu::malloc()]]). As a workaround, such deallocators could be defined (C99) inline, so that the analyzer can see that they are internally calling the actual deallocator, but that's hard when the deallocator is in a library, which might support C89, as is probably the case in libbsd.
Oops, no, that's a different story. The analyzer is thinking it leaks somewhere where it doesn't seem to leak. The false positive still reproduces with gcc-14 (Debian 14-20240429-1) 14.0.1 20240429 (experimental) [gcc-14 r14-10144-g41d7a8ceaaa]
Here's a smaller reproducer: $ cat pass.c #include <stdlib.h> void my_free(char *p); [[gnu::malloc(my_free)]] char *my_malloc(void); int main(void) { char *p; p = my_malloc(); my_free(p); // 2 false positives. } char *my_malloc(void) { return malloc(42); } void my_free(char *p) { free(p); } $ gcc-14 -Wall -Wextra pass.c -fanalyzer -O3 pass.c: In function ‘main’: pass.c:10:9: warning: ‘p’ should have been deallocated with ‘free’ but was deallocated with ‘my_free’ [CWE-762] [-Wanalyzer-mismatching-deallocation] 10 | my_free(p); // 2 false positives. | ^~~~~~~~~~ ‘main’: events 1-2 | | 6 | int main(void) | | ^~~~ | | | | | (1) entry to ‘main’ |...... | 9 | p = my_malloc(); | | ~~~~~~~~~~~ | | | | | (2) calling ‘my_malloc’ from ‘main’ | +--> ‘my_malloc’: events 3-4 | | 13 | char *my_malloc(void) | | ^~~~~~~~~ | | | | | (3) entry to ‘my_malloc’ | 14 | { | 15 | return malloc(42); | | ~~~~~~~~~~ | | | | | (4) allocated here (expects deallocation with ‘free’) | <------+ | ‘main’: events 5-6 | | 9 | p = my_malloc(); | | ^~~~~~~~~~~ | | | | | (5) returning to ‘main’ from ‘my_malloc’ | 10 | my_free(p); // 2 false positives. | | ~~~~~~~~~~ | | | | | (6) deallocated with ‘my_free’ here; allocation at (4) expects deallocation with ‘free’ | pass.c: In function ‘my_malloc’: pass.c:15:16: warning: leak of ‘p’ [CWE-401] [-Wanalyzer-malloc-leak] 15 | return malloc(42); | ^~~~~~~~~~ ‘main’: events 1-3 | | 6 | int main(void) | | ^~~~ | | | | | (1) entry to ‘main’ |...... | 9 | p = my_malloc(); | | ~~~~~~~~~~~ | | | | | (2) allocated here | | (3) calling ‘my_malloc’ from ‘main’ | +--> ‘my_malloc’: events 4-5 | | 13 | char *my_malloc(void) | | ^~~~~~~~~ | | | | | (4) entry to ‘my_malloc’ | 14 | { | 15 | return malloc(42); | | ~~~~~~~~~~ | | | | | (5) ‘p’ leaks here; was allocated at (2) |