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SubjectRe: Memory corruption due to word sharing
On Wed, Feb 1, 2012 at 12:01 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
>  - However, while using the *smallest* possible access may generate
> correct code, it often generates really *crappy* code. Which is
> exactly the bug that I reported in
>
>   http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48696

Btw, as I also pointed out in that (old) bugzilla entry, gcc can do
pretty much whatever it damn well pleases with loads from a bitfield.

You can narrow them, you can make them wider, you can paint them pink.

Widening the loads can still be a performance problem, and technically
it's a really bad idea for any nearby "volatile"s too, but in practice
things will *work*.

Writes are much more critical. If you overwrite a field that is no
longer in the bitfield, you can no longer depend on "nobody cares
about bitfield accesses", because by definition you are clearly now
stepping on non-bitfield things.

You do realize that even in user space, and even before C11, people
have things like "sig_atomic_t" etc. So even if you don't like the
notion of "volatile", here's *another* example of this being real gcc
bug:

struct mydata {
sig_atomic_t seen_signal;
unsigned flags:1;
};

and now do the same test-program, realizing that "sig_atomic_t" is
normally the exact same thing as "int".

Now, thing about what happens if you have a signal handler that comes
in and does

mydata.seen_signal = 1;

and happens to interrupt the code that changes "mydata.flags" in just
the right spot.

That's right: the bitfield update will possibly *clear* that
"signal-safe" flag again, and gcc just created buggy asm code from
correct C code.

Guys, if you don't admit that this is a bug, I don't know what to say.

IT IS A GCC BUG.

Don't try to make it into something bigger and related to C++11/C11.
Don't try to talk about "memory models". Just admit that it is a bug
to do a 64-bit write to a 32-bit bitfield, and fix it!

Linus
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