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Date: Thu, 14 Jul 2011 01:44:59 +0400
From: Solar Designer <solar@...nwall.com>
To: musl@...ts.openwall.com
Subject: Re: cluts review
On Thu, Jul 14, 2011 at 12:39:45AM +0400, Solar Designer wrote:
> Rich might be able to provide a better / more correct answer (I'd be
> interested), but here's my understanding:
>
> The clobbering happens when those variables are kept in registers rather
> than in memory (on stack). To prevent it from happening, you may force
> the compiler not to place the variables in registers. One way to do it
> is to take the variable's address:
>
> (void) &var;
>
> (I am not sure what guarantees this provides. IIRC, it was documented
> to provide the needed safety under GNU C.)
I can't find where I think I previously saw this documented for gcc.
> Another is to declare it "volatile", but this does a bit more than is
> needed (so has extra performance impact).
This is documented in lots of places. For example:
https://www.securecoding.cert.org/confluence/display/seccode/MSC22-C.+Use+the+setjmp(),+longjmp()+facility+securely
and indeed we're taking a risk by potentially interrupting and
re-entering non-async-signal safe functions:
https://www.securecoding.cert.org/confluence/display/seccode/SIG32-C.+Do+not+call+longjmp%28%29+from+inside+a+signal+handler
IIRC, I brought this up before, and Rich correctly pointed out that we
were doing it for low-risk functions only, and this is only a test suite.
> Better yet, structure your function such that there are no variables to
> clobber. If you put your sigsetjmp() at the very beginning of the
> function, before any local variable is assigned a value, there's nothing
> to clobber yet.
Here's an example:
http://cvsweb.openwall.com/cgi/cvsweb.cgi/Owl/packages/popa3d/popa3d/protocol.c?rev=HEAD
int pop_handle_state(struct pop_command *commands)
{
char line[POP_BUFFER_SIZE];
char *params;
struct pop_command *command;
int response;
if (sigsetjmp(pop_timed_out, 1)) return POP_CRASH_NETTIME;
while (pop_get_line(line, sizeof(line))) {
...
It also invokes siglongjmp() from a signal handler, but it only installs
the handler for the duration of a read() syscall, which is async signal
safe, so there's no risk.
Alexander
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