linux-kernel - Re: [RFC PATCH 6/7] x86/kexec: Debugging support: Dump registers on exception

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Message-ID: <66DC35BB-A9E5-45E4-AAAD-051C22943F51@zytor.com>
Date: Tue, 05 Nov 2024 12:50:56 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: "Woodhouse, David" <dwmw@...zon.co.uk>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "kexec@...ts.infradead.org" <kexec@...ts.infradead.org>,
        "jpoimboe@...nel.org" <jpoimboe@...nel.org>
CC: "horms@...nel.org" <horms@...nel.org>, "x86@...nel.org" <x86@...nel.org>,
        "bp@...en8.de" <bp@...en8.de>, "mingo@...hat.com" <mingo@...hat.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "kai.huang@...el.com" <kai.huang@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "nik.borisov@...e.com" <nik.borisov@...e.com>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>
Subject: Re: [RFC PATCH 6/7] x86/kexec: Debugging support: Dump registers on exception

On November 5, 2024 12:38:10 PM PST, "Woodhouse, David" <dwmw@...zon.co.uk> wrote:
>On Sun, 2024-11-03 at 05:35 +0000, David Woodhouse wrote:
>> 
>> +
>> +/* Print the byte in %bl, clobber %rax */
>> +SYM_CODE_START_LOCAL_NOALIGN(pr_byte)
>> +       movb    %bl, %al
>> +       nop
>> +       andb    $0x0f, %al
>> +       addb    $0x30, %al
>> +       cmpb    $0x3a, %al
>> +       jb      1f
>> +       addb    $('a' - '0' - 10), %al
>> +1:     pr_char
>> +       ANNOTATE_UNRET_SAFE
>> +       ret
>> +SYM_CODE_END(pr_byte)
>> +
>
>Obviously that function name (and comment) are wrong; fixed in my tree.
>at
>https://git.infradead.org/users/dwmw2/linux.git/shortlog/refs/heads/kexec-debug
>
>This function (and also pr_qword) are also what objtool is complaining
>about:
>
>vmlinux.o: warning: objtool: relocate_range+0x2f6: unreachable instruction
>vmlinux.o: warning: objtool: relocate_range+0x305: unreachable instruction
>
>I don't quite see why, because pr_qword() quite blatantly calls
>pr_nyblle(), as it's now named. And exc_handler() repeatedly calls
>pr_qword().
>
>But most of the objtool annotations I've added here were just to make
>it shut up and build, without much though. Peter, Josh, any chance you
>can help me fix it up please?
>
>It would also be really useful if objtool would let me have data inside
>a "code" segment, without complaining that it can't decode it as
>instructions — and without also failing to decode the first instruction
>of the *subsequent* function. I've put the GDT at the end to work
>around that, but it's a bit nasty.

code in the data *section* or *segment*? Either is nasty, though. That's what .rodata is for.