linux-kernel - Re: Problems with commit 'kallsyms: add support for relative offsets in kallsyms address table' (in mmotm)

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Date:	Sun, 24 Jan 2016 09:21:04 +0100
From:	Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	Heiko Carstens <heiko.carstens@...ibm.com>,
	Michael Ellerman <mpe@...erman.id.au>,
	Kees Cook <keescook@...omium.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Problems with commit 'kallsyms: add support for relative offsets
 in kallsyms address table' (in mmotm)

On 24 January 2016 at 08:06, Guenter Roeck <linux@...ck-us.net> wrote:
> On 01/23/2016 10:10 PM, Ard Biesheuvel wrote:
>>
>>
>>
>>> On 24 jan. 2016, at 03:35, Guenter Roeck <linux@...ck-us.net> wrote:
>>>
>>>> On 01/23/2016 06:06 PM, Guenter Roeck wrote:
>>>> Hi,
>>>>
>>>> I see runtime problems with the current mmotm branch. All qemu mips
>>>> targets
>>>> (32 and 64 bit, big and little endian) are stuck in boot after this
>>>> commit.
>>>>
>>>> Bisect points to commit d13682e4d9d2 ("kallsyms: add support for
>>>> relative offsets
>>>> in kallsyms address table". Disabling CONFIG_KALLSYMS_BASE_RELATIVE
>>>> fixes the problem,
>>>> ie I can boot the image with qemu.
>>>>
>>>> Bisect log is attached.
>>>>
>>>> Playing with the problem, I found the following:
>>>>
>>>> 1) The problem is only seen with a toolchain using binutils 2.22, but
>>>> not
>>>>     with a toolchain using binutils 2.25. The compiler configuration may
>>>> be
>>>>     different for both toolchains.
>>>> 2) Message "kallsyms failure: absolute symbol value 0xffffffff807afd14
>>>> out of range
>>>>     in relative mode" (twice) when using the toolchain with binutils
>>>> 2.22.
>>>>     This does not cause the build to fail, though.
>>>> 3) kallsyms_sym_address() parameter variable type is "int". In the
>>>> calling code,
>>>>     the variable type used is "unsigned long". That has no impact on the
>>>> problem,
>>>>     though.
>>>
>>>
>>> An additional data point: When using the older toolchain, many symbols in
>>> System.map
>>> are marked "A".
>>>     ffffffff80100000 A _text
>>> With the more recent toolchain, the same symbols are marked "T".
>>>     ffffffff80100000 T _text
>>>
>>
>> Thanks for the analysis. It is surprising that the build does not fail
>> when this occurs, and the subsequent hangs themselves are probably caused by
>> missing kallsyms data.
>>
> Yes, I wondered why the build doesn't fail. Seems odd.
>
>> scripts/kallsyms.c ignores all A symbols except _text, which is actually a
>> relative symbol by nature so we can simply assume it is relative (i.e.,
>> override it as T)
>>
>> Re x86_64 !SMP, any build time errors there as well? Likewise for sparc32?
>>
>
> Yes, same kind of errors for both. For x86_64/nosmp I also get the error
> message
> when using the Ubuntu native toolchain, so it doesn't seem to be (directly)
> related to binutils 2.22 vs. 2.25 for that architecture.
>
> Runtime behavior is a bit different for the different architectures.
> x86_64 dies silently without any console output, mips just hangs,
> and sparc32 gets a panic with NULL pointer access.
> Of course, with missing kallsyms data all bets are off.
>
>>
>> Thanks again, and sorry for the trouble,
>
>
> No worries. Hope you'll get this sorted out.
>

OK, there's an additional issue in my latest version: the
kallsyms_relative_base value itself is not relocated.

If you have more time to burn on this, could you try the following on
top? (If not, that is also fine, I will look into it myself on Monday)

diff --git a/scripts/kallsyms.c b/scripts/kallsyms.c
index 5ab13394dfd9..0f43f0751d47 100644
--- a/scripts/kallsyms.c
+++ b/scripts/kallsyms.c
@@ -137,8 +137,10 @@ static int read_symbol(FILE *in, struct sym_entry *s)
                sym++;

        /* Ignore most absolute/undefined (?) symbols. */
-       if (strcmp(sym, "_text") == 0)
+       if (strcmp(sym, "_text") == 0) {
                _text = s->addr;
+               stype = 'T';
+       }
        else if (check_symbol_range(sym, s->addr, text_ranges,
                                    ARRAY_SIZE(text_ranges)) == 0)
                /* nothing to do */;
@@ -406,7 +408,7 @@ static void write_src(void)

        if (base_relative) {
                output_label("kallsyms_relative_base");
-               printf("\tPTR\t%#llx\n", relative_base);
+               printf("\tPTR\t_text - %#llx\n", _text - relative_base);
                printf("\n");
        }


Thanks,
Ard.