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Description of problem:
libselinux matchpathcon() calls realpath_not_final() function if the path is a symlink.
realpath_not_final doesn't track double slash properly that caused to free() on invalid pointer.


How reproducible:
always

Steps to Reproduce:
1. matchpathcon /bin ( it works )
2. matchpathcon //bin ( it crashes )
3. or compile and run the following code:
#include <selinux/selinux.h>
#include <sys/stat.h>
#include <stdlib.h>

int main() {
    char **conptr = malloc(sizeof(char *));
    struct stat st;
    lstat("//bin", &st);
    matchpathcon("//bin", st.st_mode, conptr);
    return 0;
}

Actual results:
*** Error in `./test': free(): invalid pointer: 0x0000000000b12f41 ***
======= Backtrace: =========
/lib64/libc.so.6(+0x7c503)[0x7feac6bd3503]
/lib64/libselinux.so.1(realpath_not_final+0xc0)[0x7feac6f2dd50]
/lib64/libselinux.so.1(matchpathcon+0xce)[0x7feac6f2deee]
./test[0x400770]
/lib64/libc.so.6(__libc_start_main+0xf5)[0x7feac6b78b35]
./test[0x400669]

Expected results:
//bin system_u:object_r:bin_t

Additional info:

I've tested latest master branch of libselinux and looks like the bug was fixed there.

This bug is reproducible on RHEL-7.3, but it is not reproducible on RHEL-7.4.

I believe this bug is a duplicate of BZ#1404644.

*** This bug has been marked as a duplicate of bug 1404644 ***