From: Peter Zijlstra <peterz@infradead.org>
To: "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>
Cc: Marc Zyngier <maz@kernel.org>, "aik@ozlabs.ru" <aik@ozlabs.ru>,
Sathvika Vasireddy <sv@linux.vnet.ibm.com>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Chen Zhongjin <chenzhongjin@huawei.com>,
"mingo@redhat.com" <mingo@redhat.com>,
Sathvika Vasireddy <sv@linux.ibm.com>,
"jpoimboe@redhat.com" <jpoimboe@redhat.com>,
"paulus@samba.org" <paulus@samba.org>,
"mbenes@suse.cz" <mbenes@suse.cz>,
"linuxppc-dev@lists.ozlabs.org" <linuxppc-dev@lists.ozlabs.org>,
"rostedt@goodmis.org" <rostedt@goodmis.org>,
Linux ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: [RFC PATCH v3 11/12] powerpc: Remove unreachable() from WARN_ON()
Date: Mon, 4 Jul 2022 13:43:15 +0200 [thread overview]
Message-ID: <YsLSU6idNME/BtwH@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <1656583960.0nqsj977sr.naveen@linux.ibm.com>
On Thu, Jun 30, 2022 at 04:07:47PM +0530, Naveen N. Rao wrote:
> Objtool classifies 'ud2' as INSN_BUG, and 'int3' as INSN_TRAP. In x86 BUG(),
Yes, ud2 is the traditional 'kill' instruction and a number of emulators
treat it as such, however it also being the shortest encoding (2 bytes)
for #UD Linux has opted to (ab)use it to implement WARN/BUG.
As such interpretation of 'ud2' needs to assume control flow stops
(compiler will also emit ud2 in a number of cases with that intent).
However, if it's used as WARN we then need to annotate the thing to not
be terminal.
> there is no need for an annotation since objtool assumes that 'ud2'
> terminates control flow. But, for __WARN_FLAGS(), since 'ud2' is used, an
> explicit annotate_reachable() is needed. That's _reachable_, to indicate
> that the control flow can continue with the next instruction.
>
> On powerpc, we should (eventually) classify all trap variants as INSN_TRAP.
Careful.. INSN_TRAP is mostly used for purposes of speculation stop and
padding. That is, INSN_TRAP does indeed not affect control flow, but the
way objtool treats it might not be quite what you want.
Specifically, straight-line-speculation checks want INT3 after indirect
control transfers (indirect jump and return -- indirect call is
'difficult'); these locations are architecturally not executed and as
such placing a random trap instruction there is 'harmless'. Of course,
were the branch predictor to go wobbly and attempt to execute it, the
fact that it's a trap will stop speculation dead.
Additionally, int3, being a single byte instruction, is also used to
fill dead code space, any #BP trap on it will not have a descriptor and
mostly cause the kernel to go splat.
Per the last usage, validate_reachable_instructions() will ignore it.
I'm not sure you want to always ignore all your (unreachable) trap
instructions.
next prev parent reply other threads:[~2022-07-04 11:45 UTC|newest]
Thread overview: 41+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-06-24 18:32 [RFC PATCH v3 00/12] objtool: Enable and implement --mcount option on powerpc Sathvika Vasireddy
2022-06-24 18:32 ` [RFC PATCH v3 01/12] objtool: Fix SEGFAULT Sathvika Vasireddy
2022-07-08 15:10 ` Christophe Leroy
2022-06-24 18:32 ` [RFC PATCH v3 02/12] objtool: Use target file endianness instead of a compiled constant Sathvika Vasireddy
2022-06-24 18:32 ` [RFC PATCH v3 03/12] objtool: Use target file class size " Sathvika Vasireddy
2022-07-08 17:35 ` Christophe Leroy
2022-06-24 18:32 ` [RFC PATCH v3 04/12] objtool: Add --mnop as an option to --mcount Sathvika Vasireddy
2022-06-24 18:32 ` [RFC PATCH v3 05/12] powerpc: Skip objtool from running on VDSO files Sathvika Vasireddy
2022-06-24 18:32 ` [RFC PATCH v3 06/12] objtool: Read special sections with alts only when specific options are selected Sathvika Vasireddy
2022-06-24 18:32 ` [RFC PATCH v3 07/12] objtool: Use macros to define arch specific reloc types Sathvika Vasireddy
2022-07-04 11:14 ` Peter Zijlstra
2022-07-04 15:53 ` Christophe Leroy
2022-07-04 16:18 ` Peter Zijlstra
2022-06-24 18:32 ` [RFC PATCH v3 08/12] objtool: Add arch specific function arch_ftrace_match() Sathvika Vasireddy
2022-06-24 18:32 ` [RFC PATCH v3 09/12] objtool/powerpc: Enable objtool to be built on ppc Sathvika Vasireddy
2022-06-24 18:32 ` [RFC PATCH v3 10/12] objtool/powerpc: Add --mcount specific implementation Sathvika Vasireddy
2022-06-24 18:32 ` [RFC PATCH v3 11/12] powerpc: Remove unreachable() from WARN_ON() Sathvika Vasireddy
2022-06-25 6:46 ` Christophe Leroy
2022-06-27 15:21 ` Sathvika Vasireddy
2022-06-27 15:35 ` Sathvika Vasireddy
2022-06-27 15:46 ` Christophe Leroy
2022-06-29 18:30 ` Christophe Leroy
2022-06-30 8:05 ` Naveen N. Rao
2022-06-30 9:58 ` Christophe Leroy
2022-06-30 10:33 ` Christophe Leroy
2022-06-30 10:37 ` Naveen N. Rao
2022-06-30 15:58 ` Segher Boessenkool
2022-07-04 12:01 ` Peter Zijlstra
2022-07-04 11:43 ` Peter Zijlstra [this message]
2022-07-01 2:13 ` Chen Zhongjin
2022-07-01 6:56 ` Sathvika Vasireddy
2022-07-01 11:40 ` [RFC PATCH v3 11/12] powerpc: Remove unreachable() from WARN_ON() (gcc issue ?) Christophe Leroy
2022-07-04 11:45 ` [RFC PATCH v3 11/12] powerpc: Remove unreachable() from WARN_ON() Peter Zijlstra
2022-07-04 12:34 ` Christophe Leroy
2022-07-05 15:48 ` Segher Boessenkool
2022-07-04 12:05 ` Peter Zijlstra
2022-07-04 12:44 ` Christophe Leroy
2022-07-04 14:19 ` Peter Zijlstra
2022-06-24 18:32 ` [RFC PATCH v3 12/12] objtool/powerpc: Fix unannotated intra-function call warnings Sathvika Vasireddy
2022-07-08 15:06 ` [RFC PATCH v3 00/12] objtool: Enable and implement --mcount option on powerpc Christophe Leroy
2022-07-08 15:42 ` Christophe Leroy
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