On 1/25/2017 6:23 AM, Torvald Riegel wrote:
I agree with Florian that the compiler needs to be made aware that
getcontext can return twice, or something to that effect. This would
tell it that it has to reason about the lifetimes of variables
differently.
The problem is that "returns_twice" doesn't offer the semantics we want.
It ensures that register-allocated variables are handled properly, i.e.
everything is saved to the stack frame prior to calling the function. But
here the issue is that the stack frame itself isn't being set up in a way that
actually works. And in practice, tagging getcontext and swapcontext with
attribute((returns_twice)) does not fix the bug. (It does seem like doing so
isn't a bad idea, but it is beyond the scope of fixing this one test bug.)
Another way to fix the problem is to make the context variables function static,
which should forbid the compiler from doing anything funky with them.
(Although do_test itself is static, it is called from main, and the compiler
has to assume main could get called again and expect to find the updated
context variables still updated, so it can't trickily ignore the static modifier
or anything like that, I think.)
commit 5a054bf335e350e96e2a38b5d2573f4f26a2185a
Author: Chris Metcalf <cmetcalf@mellanox.com>
Date: Fri Jan 13 12:50:50 2017 -0500
tst-setcontext2: avoid bug from compiler optimization
With an uninitialized oldctx, the compiler is free to observe that
the only path that sets up a value in oldctx is through the
"if (global == 2)" arm, in which arm we apparently return 0 without
referencing oldctx again.
Then, after the "if" cascade, the compiler can inline the "check"
function and then observe that the sigset_t "set" variable there
is only used locally, before any apparent uses of oldctx, and as a
result it can decide to use the same stack region for both variables.
Unfortunately this has the effect of clobbering oldctx when we call
sigprocmask, and results in the test failing.
By making oldctx (and ctx) have static scope, we forbid the compiler
from performing this optimization. The compiler will be required
to allocate them to separate memory so that they would have their
updated values if the do_test function were to be invoked a second
time. (We know that doesn't happen, but the compiler can't prove
it solely by examination of this compilation unit.)
Seen on tilegx with gcc 4.8 at -O3.
diff --git a/stdlib/tst-setcontext2.c b/stdlib/tst-setcontext2.c
index 07fb974c4684..c937f6c396db 100644
--- a/stdlib/tst-setcontext2.c
+++ b/stdlib/tst-setcontext2.c
@@ -87,7 +87,7 @@ handler (int __attribute__ ((unused)) signum)
static int
do_test (void)
{
- ucontext_t ctx, oldctx;
+ static ucontext_t ctx, oldctx;
struct sigaction action;
pid_t pid;