From: Martin Uecker <uecker@tugraz.at>
To: Kees Cook <keescook@chromium.org>
Cc: Qing Zhao <qing.zhao@oracle.com>,
Siddhesh Poyarekar <siddhesh@gotplt.org>,
Richard Biener <richard.guenther@gmail.com>,
Joseph Myers <joseph@codesourcery.com>,
Jakub Jelinek <jakub@redhat.com>,
gcc Patches <gcc-patches@gcc.gnu.org>,
"isanbard@gmail.com" <isanbard@gmail.com>
Subject: Re: HELP: Will the reordering happen? Re: [V3][PATCH 0/3] New attribute "counted_by" to annotate bounds for C99 FAM(PR108896)
Date: Thu, 26 Oct 2023 18:45:37 +0200 [thread overview]
Message-ID: <6e07e4da210b3ee53e3bae5b18949a9d62b2a0b0.camel@tugraz.at> (raw)
In-Reply-To: <202310260851.B3719E6928@keescook>
Am Donnerstag, dem 26.10.2023 um 09:13 -0700 schrieb Kees Cook:
> On Thu, Oct 26, 2023 at 10:15:10AM +0200, Martin Uecker wrote:
> > but not this:
> >
x->count = 11;
> > char *p = &x->buf;
> > x->count = 1;
> > p[10] = 1; // !
>
> This seems fine to me -- it's how I'd expect it to work: "10" is beyond
> "1".
Note that the store would be allowed.
>
> > (because the pointer is passed around the
> > store to the counter)
> >
> > and also here the second store is then irrelevant
> > for the access:
> >
> > x->count = 10;
> > char* p = &x->buf;
> > ...
> > x->count = 1; // somewhere else
> > ----
> > p[9] = 1; // ok, because count matter when buf was accesssed.
>
> This is less great, but I can understand why it happens. "p" loses the
> association with "x". It'd be nice if "p" had to way to retain that it
> was just an alias for x->buf, so future p access would check count.
The problem is not to discover that p is an alias to x->buf,
but that it seems difficult to make sure that stores to
x->count are not reordered relative to the final access to
p[i] you want to check, so that you then get the right value.
>
> But this appears to be an existing limitation in other areas where an
> assignment will cause the loss of object association. (I've run into
> this before.) It's just more surprising in the above example because in
> the past the loss of association would cause __bdos() to revert back to
> "SIZE_MAX" results ("I don't know the size") rather than an "outdated"
> size, which may get us into unexpected places...
>
> > IMHO this makes sense also from the user side and
> > are the desirable semantics we discussed before.
> >
> > But can you take a look at this?
> >
> >
> > This should simulate it fairly well:
> > https://godbolt.org/z/xq89aM7Gr
> >
> > (the call to the noinline function would go away,
> > but not necessarily its impact on optimization)
>
> Yeah, this example should be a very rare situation: a leaf function is
> changing the characteristics of the struct but returning a buffer within
> it to the caller. The more likely glitch would be from:
>
> int main()
> {
> struct foo *f = foo_alloc(7);
> char *p = FAM_ACCESS(f, size, buf);
>
> printf("%ld\n", __builtin_dynamic_object_size(p, 0));
> test1(f); // or just "f->count = 10;" no function call needed
> printf("%ld\n", __builtin_dynamic_object_size(p, 0));
>
> return 0;
> }
>
> which reports:
> 7
> 7
>
> instead of:
> 7
> 10
>
> This kind of "get an alias" situation is pretty common in the kernel
> as a way to have a convenient "handle" to the array. In the case of a
> "fill the array without knowing the actual final size" code pattern,
> things would immediately break:
>
> struct foo *f;
> char *p;
> int i;
>
> f = alloc(maximum_possible);
> f->count = 0;
> p = f->buf;
>
> for (i; data_is_available() && i < maximum_possible; i++) {
> f->count ++;
> p[i] = next_data_item();
> }
>
> Now perhaps the problem here is that "count" cannot be used for a count
> of "logically valid members in the array" but must always be a count of
> "allocated member space in the array", which I guess is tolerable, but
> isn't ideal -- I'd like to catch logic bugs in addition to allocation
> bugs, but the latter is certainly much more important to catch.
Maybe we could have a warning when f->buf is not directly
accessed.
Martin
>
next prev parent reply other threads:[~2023-10-26 16:45 UTC|newest]
Thread overview: 116+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-08-25 15:24 Qing Zhao
2023-08-25 15:24 ` [V3][PATCH 1/3] Provide counted_by attribute to flexible array member field (PR108896) Qing Zhao
2023-09-08 14:12 ` Qing Zhao
2023-09-20 13:44 ` Ping * 2: " Qing Zhao
2023-10-05 18:51 ` Siddhesh Poyarekar
2023-10-05 19:31 ` Siddhesh Poyarekar
2023-10-18 14:51 ` Qing Zhao
2023-10-18 15:18 ` Siddhesh Poyarekar
2023-10-18 15:37 ` Qing Zhao
2023-10-18 14:41 ` Qing Zhao
2023-08-25 15:24 ` [V3][PATCH 2/3] Use the counted_by atribute info in builtin object size [PR108896] Qing Zhao
2023-09-08 14:12 ` Qing Zhao
2023-09-20 13:44 ` PING *2: " Qing Zhao
2023-10-05 20:01 ` Siddhesh Poyarekar
2023-10-18 20:39 ` Qing Zhao
2023-08-25 15:24 ` [V3][PATCH 3/3] Use the counted_by attribute information in bound sanitizer[PR108896] Qing Zhao
2023-09-08 14:12 ` Qing Zhao
2023-09-20 13:45 ` PING * 2: " Qing Zhao
2023-08-25 19:51 ` [V3][PATCH 0/3] New attribute "counted_by" to annotate bounds for C99 FAM(PR108896) Kees Cook
2023-09-08 14:11 ` Qing Zhao
2023-09-20 13:43 ` PING * 2: " Qing Zhao
2023-10-05 20:08 ` Siddhesh Poyarekar
2023-10-05 22:35 ` Kees Cook
2023-10-06 5:11 ` Martin Uecker
2023-10-06 10:50 ` Siddhesh Poyarekar
2023-10-06 20:01 ` Martin Uecker
2023-10-18 15:37 ` Siddhesh Poyarekar
2023-10-18 19:35 ` Qing Zhao
2023-10-18 21:11 ` Qing Zhao
2023-10-19 23:33 ` Kees Cook
2023-10-20 9:50 ` Martin Uecker
2023-10-20 18:34 ` Kees Cook
2023-10-20 18:48 ` Qing Zhao
2023-10-20 19:54 ` Martin Uecker
2023-10-23 18:17 ` Qing Zhao
2023-10-23 19:52 ` Kees Cook
2023-10-23 19:57 ` Martin Uecker
2023-10-23 22:03 ` Kees Cook
2023-10-20 17:08 ` HELP: Will the reordering happen? " Qing Zhao
2023-10-20 18:22 ` Richard Biener
2023-10-20 18:38 ` Qing Zhao
2023-10-20 19:10 ` Siddhesh Poyarekar
2023-10-20 20:41 ` Qing Zhao
2023-10-23 7:57 ` Richard Biener
2023-10-23 11:27 ` Siddhesh Poyarekar
2023-10-23 12:34 ` Richard Biener
2023-10-23 13:23 ` Siddhesh Poyarekar
2023-10-23 15:14 ` Qing Zhao
2023-10-23 14:56 ` Qing Zhao
2023-10-23 15:57 ` Richard Biener
2023-10-23 16:37 ` Qing Zhao
2023-10-23 18:06 ` Martin Uecker
2023-10-23 18:31 ` Martin Uecker
2023-10-23 19:00 ` Qing Zhao
2023-10-23 19:37 ` Martin Uecker
2023-10-23 20:33 ` Qing Zhao
2023-10-23 18:33 ` Qing Zhao
2023-10-23 18:43 ` Siddhesh Poyarekar
2023-10-23 18:55 ` Martin Uecker
2023-10-23 19:43 ` Qing Zhao
2023-10-23 22:48 ` Siddhesh Poyarekar
2023-10-24 20:30 ` Qing Zhao
2023-10-24 20:38 ` Martin Uecker
2023-10-24 21:09 ` Siddhesh Poyarekar
2023-10-24 22:51 ` Qing Zhao
2023-10-24 23:56 ` Siddhesh Poyarekar
2023-10-25 13:27 ` Qing Zhao
2023-10-25 14:50 ` Siddhesh Poyarekar
2023-10-25 15:38 ` Richard Biener
2023-10-25 19:03 ` Qing Zhao
2023-10-26 5:21 ` Jakub Jelinek
2023-10-26 8:56 ` Richard Biener
2023-10-26 14:58 ` Qing Zhao
2023-10-26 15:48 ` Richard Biener
2023-10-26 16:16 ` Martin Uecker
2023-10-26 14:41 ` Qing Zhao
2023-10-25 18:44 ` Qing Zhao
2023-10-25 22:06 ` Kees Cook
2023-10-25 22:27 ` Qing Zhao
2023-10-25 22:32 ` Kees Cook
2023-10-26 8:15 ` Martin Uecker
2023-10-26 16:13 ` Kees Cook
2023-10-26 16:45 ` Martin Uecker [this message]
2023-10-26 19:57 ` Qing Zhao
2023-10-27 7:21 ` Martin Uecker
2023-10-27 14:32 ` Qing Zhao
2023-10-27 14:53 ` Martin Uecker
2023-10-27 15:10 ` Qing Zhao
2023-10-27 17:19 ` Kees Cook
2023-10-27 18:13 ` Qing Zhao
2023-10-25 5:26 ` Martin Uecker
2023-10-25 6:43 ` Richard Biener
2023-10-25 8:16 ` Martin Uecker
2023-10-25 10:25 ` Siddhesh Poyarekar
2023-10-25 10:47 ` Martin Uecker
2023-10-25 11:13 ` Richard Biener
2023-10-25 18:16 ` Martin Uecker
2023-10-26 8:45 ` Richard Biener
2023-10-26 9:20 ` Martin Uecker
2023-10-26 10:14 ` Martin Uecker
2023-10-26 14:05 ` Richard Biener
2023-10-26 18:54 ` Qing Zhao
2023-10-27 16:43 ` Qing Zhao
2023-10-26 16:41 ` Qing Zhao
2023-10-26 17:05 ` Martin Uecker
2023-10-26 17:35 ` Richard Biener
2023-10-26 19:20 ` Qing Zhao
2023-10-25 18:17 ` Qing Zhao
2023-10-25 10:25 ` Richard Biener
2023-10-25 10:39 ` Martin Uecker
2023-10-25 18:06 ` Qing Zhao
2023-10-24 21:03 ` Siddhesh Poyarekar
2023-10-24 22:41 ` Qing Zhao
2023-10-24 23:51 ` Siddhesh Poyarekar
2023-10-25 21:59 ` Kees Cook
2023-10-23 18:10 ` Joseph Myers
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