util_file_readable does not check what one expects it to check

[valentin-krasontovitsch]

util_file_readable uses util_entry_readable (and util_is_file, not focussed on here) - that function, util_entry_readable, looks as follows:

bool util_entry_readable(const char *entry) {                                                                                                                                                         
    stat_type buffer;                                                                                                                                                         
    if (util_stat(entry, &buffer) == 0)                                                                                                                                                                               
        return buffer.st_mode & S_IRUSR;                                                                                                                                                                                       
    else                                                                                                                                                         
        return false;  
}

we're getting the stat of the file, and check the mode for S_IRUSR bit being set.
from the glibc docs:

S_IREAD
Read permission bit for the owner of the file. On many systems this bit is 0400.
S_IREAD is an obsolete synonym provided for BSD compatibility.

so we only check whether the owner of the file can read the file.

this check returns true, say for a file with mode 0400 and owner root:root, but cannot be read by a non-root user.