Silence a warning.

Once upon a time you could call fchown() and let it fail. Then gcc decided
not using its return code was inconcievable, but you could typecast it to (void)
to shut it up. Then gcc noticed people doing that and clutched its pearls and
took it away, so I added an if() statement that does nothing with the result
because we _expect_ this to fail when we're not root. Then clang started
complaining about an if (); statement with the semicolon on the same line,
but decided it's ok if the ; is on the next line (I.E. significant whitespace
in C), so I'm adding an "assignemnt to self" that gets optimized away so
it does a more _explicit_ nothing (the same way you suppress gcc's broken
"this isn't used uninitialized" warnings).

If the compilers weren't going to so much trouble to force the issue I might
add code to only call fchown when we're UID 0, but I refuse to be coerced
into it. (And if getpid() is still a system call instead of a vdso member
then it doesn't actually _save_ us anything, the dentry should be hot and
the permission check was just "if (!uid)" before selinux entered into it and
we're operating on an fd so the security's the same.)

1 files changed, 6 insertions, 3 deletions

diff --git a/lib/lib.c b/lib/lib.c
index c20a06cf..6f7ed303 100644
--- a/lib/lib.c
+++ b/lib/lib.c
@@ -645,9 +645,12 @@ int copy_tempfile(int fdin, char *name, char **tempname)
   fstat(fdin, &statbuf);
   fchmod(fd, statbuf.st_mode);
 
-  // It's fine if this fails (generally because we're not root), but gcc no
-  // longer lets a (void) typecast silence the "unused result" warning, so...
-  if (fchown(fd, statbuf.st_uid, statbuf.st_gid));
+  // We chmod before chown, which strips the suid bit. Caller has to explicitly
+  // switch it back on if they want to keep suid.
+
+  // I said IGNORING ERRORS. Both gcc and clang clutch their pearls about this
+  // but it's _supposed_ to fail when we're not root.
+  if (fchown(fd, statbuf.st_uid, statbuf.st_gid)) fd = fd;
 
   return fd;
 }