[Rpm-maint] [rpm-software-management/rpm] First steps towards fixing the symlink CVEs (PR #1919)

[Demi Marie Obenour]

@DemiMarie commented on this pull request.

On Linux, using `openat2()` will be much simpler and more efficient on kernels that support it.  RPM is not Linux-specific, but `openat2()` might be useful where available.

> @@ -103,6 +103,8 @@ option to configure).  For GCC, OpenMP 4.5 is fully supported since GCC 6.1,
 which is available from
     http://www.gnu.org/
 
+Rpm requires a POSIX.1-2008 level operating system.

```suggestion
RPM requires a POSIX.1-2008 level operating system.
```

>  	       (rc < 0 ? strerror(errno) : ""));
     if (rc < 0)	rc = RPMERR_MKDIR_FAILED;
     return rc;
 }
 
+static int fsmOpenat(int dirfd, const char *path, int flags)
+{
+    struct stat lsb, sb;
+    int sflags = flags | O_NOFOLLOW;
+    int fd = openat(dirfd, path, sflags);
+
+    /*
+     * Only ever follow symlinks by root or target owner. Since we can't
+     * open the symlink itself, the order matters: we stat the link *after*

Would Linux `O_PATH` help?

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