linux-kernel - Re: [PATCH] objtool: improve detection of BUG() and other dead ends

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Message-ID: <20170221214326.GL6515@twins.programming.kicks-ass.net>
Date:   Tue, 21 Feb 2017 22:43:26 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] objtool: improve detection of BUG() and other dead ends

On Tue, Feb 21, 2017 at 03:35:32PM -0600, Josh Poimboeuf wrote:
> The BUG() macro's use of __builtin_unreachable() via the unreachable()
> macro tells gcc that the instruction is a dead end, and that it's safe
> to assume the current code path will not execute past the previous
> instruction.
> 
> On x86, the BUG() macro is implemented with the 'ud2' instruction.  When
> objtool's branch analysis sees that instruction, it knows the current
> code path has come to a dead end.
> 
> Peter Zijlstra has been working on a patch to change the WARN macros to
> use 'ud2'.  That patch will break objtool's assumption that 'ud2' is
> always a dead end.
> 
> Generally it's best for objtool to avoid making those kinds of
> assumptions anyway.  The more ignorant it is of kernel code internals,
> the better.
> 
> So create a more generic way for objtool to detect dead ends by adding
> an annotation to the unreachable() macro.  The annotation stores a
> pointer to the end of the unreachable code path in an '__unreachable'
> section.  Objtool can read that section to find the dead ends.
> 

Tested-by: Peter Zijlstra (Intel) <peterz@...radead.org>