Adding file descriptor attribute(s) to gcc and glibc

[David Malcolm]

On Thu, 2022-07-14 at 09:30 +0100, Szabolcs Nagy wrote:
> The 07/13/2022 12:55, David Malcolm wrote:
> > On Wed, 2022-07-13 at 16:01 +0200, Florian Weimer wrote:
> > > * David Malcolm:
> > GCC trunk's -fanalyzer implements the new warnings via a state
> > machine
> > for file-descriptor values; it currently has rules for handling
> > "open",
> > "close", "read", and "write", and these functions are currently hard-
> > coded inside the analyzer.
> > 
> > Here are some examples on Compiler Explorer of what it can/cannot
> > detect:
> >   https://godbolt.org/z/nqPadvM4f
> > 
> > Probably the most important one IMHO is the leak detection.
> 
> nice.
> 
> > Would it be helpful to have some kind of attribute for "returns a new
> > open FD"?  Are there other ways to close a FD other than calling
> > "close" on it?  (Would converting that to some kind of "closes"
> > attribute be a good idea?)

Thanks, lots of good ideas here; I've filed various RFEs about these;
I'm posting the details below for reference.

> 
> dup2(oldfd, newfd)
> dup3(oldfd, newfd, flags)
> 
> closes newfd (and also opens it to be a dup of oldfd)
> unless the call fails.
dup and friends probably need special-casing; I've filed this as:
  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106298

> 
> close_range(first, last, flags)
close_range and closefrom would need special-casing, already filed as:
  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106283

> 
> fclose(fdopen(fd, mode))

The analyzer now attempts to track both file descriptors and stdio
streams, so we probably need to special-case fdopen to capture the
various possible interactions between these two leak detectors; I've
filed this as:
  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106299
with an implementation idea there.

> 
> but users can write all sorts of wrappers around close too.

Yeah.  If the -fanalyzer leak detectors see a value "escape" into
unknown code, then they don't report leaks; see e.g.:
  https://godbolt.org/z/n8fMhGTP5
where we don't report in test_2 about fd leaking due to the call to:

  might_close (fd);

which is extern, and so we conservatively assume that fd doesn't leak.

> 
> > 
> > Are there any other "magic" values for file-descriptors we should be
> > aware of?
> > 
> 
> mmap may require fd==-1 for anonymous maps.

mmap is its own can of worms, which I've filed as:
  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106301

You also reminded me that we need to track other ways in which the user
could obtain an fd that could leak, which I've filed as:
  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106300
(covering creat, pipe and friends, dup and friends, fcntl, and socket).


I've added all of these to the top-level RFE for -fanalyzer tracking
file descriptors:
  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=analyzer-fd

which is now a tracker-bug:
  https://gcc.gnu.org/bugzilla/showdependencytree.cgi?id=analyzer-fd


Thanks again for the ideas
Dave