linux-kernel - Re: [PATCH] objtool: fix another gcc jump table detection issue

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Date:   Fri, 3 Mar 2017 00:01:25 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Denys Vlasenko <dvlasenk@...hat.com>
Subject: Re: [PATCH] objtool: fix another gcc jump table detection issue

On Thu, Mar 2, 2017 at 11:57 PM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
>
> Arnd Bergmann reported a (false positive) objtool warning:
>
>   drivers/infiniband/sw/rxe/rxe_resp.o: warning: objtool: rxe_responder()+0xfe: sibling call from callable instruction with changed frame pointer
>
> The issue is in find_switch_table().  It tries to find a switch
> statement's jump table by walking backwards from an indirect jump
> instruction, looking for a relocation to the .rodata section.  In this
> case it stopped walking prematurely: the first .rodata relocation it
> encountered was for a variable (resp_state_name) instead of a jump
> table, so it just assumed there wasn't a jump table.
>
> The fix is to ignore any .rodata relocation which refers to an ELF
> object symbol.  This works because the jump tables are anonymous and
> have no symbols associated with them.
>
> Reported-by: Arnd Bergmann <arnd@...db.de>
> Fixes: 3732710ff6f2 ("objtool: Improve rare switch jump table pattern detection")
> Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>

Tested-by: Arnd Bergmann <arnd@...db.de>