Re: [PATCH] warn for unsafe calls to __builtin_return_address

[Segher Boessenkool <segher at kernel dot crashing dot org>]

On Thu, Jul 23, 2015 at 11:08:21PM -0600, Jeff Law wrote:
> >There's the following comment in expand_builtin_frame_address:
> >
> >       /* Some ports cannot access arbitrary stack frames.  */
> >
> >just before a block of code where the function can lead to
> >an "invalid argument" warning which would cause the newly
> >added tests to fail (since the newly added warning wouldn't
> >be issued).
> >
> >I tried to determine what ports these might be so I could add
> >conditionals to the tests to prevent false positives there but
> >couldn't find any.

A quick grep shows aarch64.  Also many other ports, for
__builtin_return_address.  For __builtin_frame_address there is no
non-hacky way for ports to say "I cannot access other frames"; see
mmix_dynamic_chain_address for more complaints about this.

> You have to start thinking creatively.  For example, consider ports 
> without dwarf2 unwinders :-)  Consider ports where the linker inserts 
> stubs between caller & callee for various reasons  Consider cases where 
> the next outermost frame is compiled by something other than GCC, or 
> perhaps was written in pure assembly.  Or consider the case when we're 
> currently in a signal handling frame...

Or simply the case where the caller does not have a frame pointer, and
the ABI does not provide another way to get at (the size of) stack frames.

> So, my suggestion would be to warn for any call with a nonzero value.

The current documentation says that you should only use nonzero values
for debug purposes.  A warning would help yes, how many people read the
manual after all :-)


Segher