From: Alejandro Colomar <alx.manpages@gmail.com>
To: Martin Uecker <uecker@tugraz.at>, GCC <gcc@gcc.gnu.org>,
Jonathan Wakely <jwakely.gcc@gmail.com>
Cc: "Iker Pedrosa" <ipedrosa@redhat.com>,
"Florian Weimer" <fweimer@redhat.com>,
"Paul Eggert" <eggert@cs.ucla.edu>,
"Michael Kerrisk" <mtk.manpages@gmail.com>,
"Jₑₙₛ Gustedt" <jens.gustedt@inria.fr>,
"David Malcolm" <dmalcolm@redhat.com>,
"Sam James" <sam@gentoo.org>,
"Yann Droneaud" <ydroneaud@opteya.com>
Subject: Re: Missed warning (-Wuse-after-free)
Date: Fri, 17 Feb 2023 12:35:06 +0100 [thread overview]
Message-ID: <0049730a-e28c-0e0f-8d92-695395f1ec21@gmail.com> (raw)
In-Reply-To: <2148ef80dee2a034ee531d662fc8709d26159ec5.camel@tugraz.at>
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Hi Martin,
On 2/17/23 09:12, Martin Uecker wrote:
> Am Freitag, dem 17.02.2023 um 02:04 +0100 schrieb Alejandro Colomar
>
>
>>
>> [...]
>>
>>>
>>> I'm not convinced that it's useful to the end-user to warn about
>>> the
>>> "use of q itself" case.
>>
>> I didn't quote the standard because I couldn't find it. I was
>> searching in C11,
>> and it seems that it was only implicitly Undefined Behavior, without
>> explicit
>> spelling (the value of the pointer was indeterminate, according to
>> C11).
>
> The value becomes indeterminate according to 6.2.4p2 in C11.
> An indeterminate value may be a trap representation 3.19.2
> If such a trap representation is read (the pointer, not
> just the pointed-to object), the behavior is undefined
> according to 6.2.6.1p5. So it is explitely UB and was
> already in C99 or even before.
What if the comparison is performed as uintptr_t?
You wouldn't have trap representations, would you?
Or we could even go to memcmp(3) to compare as char,
if paranoic enough :)
>
>
>> Now C23 will better clarify that reading such a pointer value (not
>> ever dereferencing) is Undefined Behavior.
>
> We did not change this for C23.
C11:
The value of a pointer becomes indeterminate when
the object it points to (or just past)
reaches the end of its lifetime.
<https://port70.net/%7Ensz/c/c11/n1570.html#6.2.4p2>
C2x (N3054 is the latest I know):
If a pointer value is used in an evaluation after
the object the pointer points to (or just past)
reaches the end of its lifetime,
the behavior is undefined.
<https://www.open-std.org/JTC1/SC22/WG14/www/docs/n3054.pdf#subsection.6.2.4>
This new wording doesn't even allow one to use memcmp(3);
just reading the pointer value, however you do it, is UB.
>
> Only the terminology regarding trap representation
> (now: non-value representation) and indeterminate
> values (now: indeterminate representation) was revised.
>
>
> There are proposal to define bevahior for such
> pointers, but I think this would be a mistake.
> (although somehow I ended up being a co-author
> of this paper),
>
> The reason is that every use of such a pointer
> risks running into sublte issues related to pointer
> provenance.
>
> So in my opinion it would be very useful to warn about
> all uses of invalid pointers, because fixing this is
> much easier than understanding and fixing provenance
> issues which might arise from incorrect use of such
> pointers.
Agree; making this defined behavior doesn't seem a good idea.
Cheers,
Alex
>
>
> Martin
--
<http://www.alejandro-colomar.es/>
GPG key fingerprint: A9348594CE31283A826FBDD8D57633D441E25BB5
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next prev parent reply other threads:[~2023-02-17 11:35 UTC|newest]
Thread overview: 39+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-02-16 14:35 Alejandro Colomar
2023-02-16 15:15 ` David Malcolm
2023-02-17 1:04 ` Alejandro Colomar
2023-02-17 1:05 ` Alejandro Colomar
2023-02-17 1:56 ` Sam James
2023-02-17 8:12 ` Martin Uecker
2023-02-17 11:35 ` Alejandro Colomar [this message]
2023-02-17 13:34 ` Andreas Schwab
2023-02-17 13:48 ` Martin Uecker
2023-02-23 19:23 ` Alex Colomar
2023-02-23 19:57 ` Martin Uecker
2023-02-24 0:02 ` Alex Colomar
2023-02-24 1:21 ` Serge E. Hallyn
2023-02-24 1:42 ` Alex Colomar
2023-02-24 3:01 ` Peter Lafreniere
2023-02-24 8:52 ` Martin Uecker
2023-02-24 8:43 ` Martin Uecker
2023-02-24 16:10 ` Serge E. Hallyn
2023-02-24 8:36 ` Martin Uecker
2023-02-24 16:01 ` Serge E. Hallyn
2023-02-24 16:37 ` Martin Uecker
2023-02-17 3:48 ` Siddhesh Poyarekar
2023-02-17 11:22 ` Alejandro Colomar
2023-02-17 13:38 ` Siddhesh Poyarekar
2023-02-17 14:01 ` Mark Wielaard
2023-02-17 14:06 ` Siddhesh Poyarekar
2023-02-17 21:20 ` [PATCH] Make -Wuse-after-free=3 the default one in -Wall Alejandro Colomar
2023-02-17 21:39 ` Siddhesh Poyarekar
2023-02-17 21:41 ` Siddhesh Poyarekar
2023-02-17 22:58 ` Alejandro Colomar
2023-02-17 23:03 ` Siddhesh Poyarekar
2023-02-17 11:24 ` Missed warning (-Wuse-after-free) Jonathan Wakely
2023-02-17 11:43 ` Alejandro Colomar
2023-02-17 12:04 ` Jonathan Wakely
2023-02-17 12:53 ` Siddhesh Poyarekar
2023-02-17 14:10 ` Jonathan Wakely
2023-02-17 13:44 ` David Malcolm
2023-02-17 14:01 ` Siddhesh Poyarekar
2023-02-17 8:49 ` Yann Droneaud
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