Re: [Bug-readline] [readline patch, gdb-7.3?] Avoid free from a signal handler [Re: [PATCH] Make interrupting tab-completion safe.]

Jan Kratochvil
2011-07-06T17:32:52Z

On Wed, 06 Jul 2011 17:58:26 +0200, Chet Ramey wrote:
> As I said, I'm willing to remove these references and see what happens. Since
> you have a way to readily reproduce the problem, I was hoping you'd do it
> and let me know what you found.

I do not think any testing matters here. This is a difficult to reproduce
race + memory corruption. While a crash proves it is wrong no crash does not
prove anything.

Even if no existing system ever crashes the code is still wrong because it
violates POSIX:
http://pubs.opengroup.org/onlinepubs/009695399/functions/xsh_chap02_04.html
The following table defines a set of functions that shall be either
reentrant or non-interruptible by signals and shall be async-signal-safe.

Static code analysis is the only valid verification. Currently the signal
code calls free() which is not listed in the safe syscalls list above,
therefore the code is not correct.

I do not know if it is possible to code _rl_handle_signal in a way which uses
only the safe syscalls and only atomic operations on volatile data structures.
Anyway even if it would be possible I find such code very fragile and
I believe the signals should be always delayed through _rl_caught_signal.

Sure it then has to depend on the application which should properly return to
readline callers immediately if it sees any EINTR.

Thanks,
Jan