msan: stat() result is seen as uninitialized (st_size)

[vstinner]

The following code fails with use-of-uninitialized-value:

#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>

int main()
{
    struct stat st;
    printf("sizeof(struct stat) = %zd\n", sizeof(struct stat));
    int fd = open(__FILE__, O_RDONLY);
    if (fd < 0) {
        return 1;
    }
    if (fstat(fd, &st)) {
        return 1;
    }
    if (st.st_size > 1) {
        return 2;
    }
    close(fd);
    return 0;
}

Output:

$ clang x.c -o x -fsanitize=memory && ./x; echo $?
sizeof(struct stat) = 144
==134123==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0x49d9e5 in main (/home/vstinner/python/main/x+0x49d9e5)
    #1 0x7f874ea5355f in __libc_start_call_main (/lib64/libc.so.6+0x2d55f)
    #2 0x7f874ea5360b in __libc_start_main@GLIBC_2.2.5 (/lib64/libc.so.6+0x2d60b)
    #3 0x41c344 in _start (/home/vstinner/python/main/x+0x41c344)

SUMMARY: MemorySanitizer: use-of-uninitialized-value (/home/vstinner/python/main/x+0x49d9e5) in main
Exiting
1

I tested on Fedora 35 with versions:

$ clang --version
clang version 13.0.0 (Fedora 13.0.0-3.fc35)
Target: x86_64-redhat-linux-gnu
Thread model: posix
InstalledDir: /usr/bin

$ rpm -q clang
clang-13.0.0-3.fc35.x86_64

Issue discovered in Python: https://bugs.python.org/issue46887

[maflcko]

This looks like a Fedora bug? I can't reproduce with the clang from https://apt.llvm.org/ or with Debian/Ubuntu system packages.

[serge-sans-paille]

Same on godbolt: https://godbolt.org/z/ovz3de65z
@vstinner you should probably report on Fedora's bugzilla then.

[vstinner]

No activity for 2 years, I close the issue.