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Date:   Tue, 8 Feb 2022 10:10:19 -0800
From:   Luis Chamberlain <mcgrof@kernel.org>
To:     Vimal Agrawal <avimalin@gmail.com>
Cc:     hch@infradead.org, masahiroy@kernel.org, michal.lkml@markovi.net,
        ndesaulniers@google.com, JBeulich@suse.com, jeffm@suse.com,
        sam@ravnborg.org, linux-kbuild@vger.kernel.org, jeyu@kernel.org,
        linux-kernel@vger.kernel.org, vimal.agrawal@sophos.com,
        nishit.shah@sophos.com, gregkh@linuxfoundation.org,
        Dirk VanDerMerwe <Dirk.VanDerMerwe@sophos.com>
Subject: Re: [PATCH v6] modules: add heuristic when stripping unneeded symbols
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References: <CALkUMdSVV=Vju3TtryObKney7Q1TJLgGd7G7OB4X3FW9eJf4-w@mail.gmail.com>
 <20220208110231.2896-1-vimal.agrawal@sophos.com>
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On Tue, Feb 08, 2022 at 04:32:31PM +0530, Vimal Agrawal wrote:
> If kernel modules are stripped off symbols for some reason 

For some reason? Are you kidding? It is by design.

> then stack
> traces in dmesg do not show symbol name for address. It just prints
> absolute address sometimes (if there is no good match with any symbol)

Yes that is by design.

> This was seen with OpenWrt which uses option INSTALL_MOD_STRIP=
> "--strip-unneeded" at kernel/module build/install time, and so modules
> are stripped off unneeded symbols.
> 
> [245864.699580]  do_nmi+0x12f/0x370
> [245864.699583]  end_repeat_nmi+0x16/0x50
> [245864.699585] RIP: 0010:0xffffffffc06b67ec                           <<<<<<<<
> [245864.699585] RSP: 0000:ffffaaa540cffe48 EFLAGS: 00000097
> [245864.699586] RAX: 0000000000000001 RBX: ffff93357a729000 RCX: 0000000000000001
> [245864.699587] RDX: ffff93357a729050 RSI: 0000000000000000 RDI: ffff93357a729000
> [245864.699588] RBP: ffff9335cf521300 R08: 0000000000000001 R09: 0000000000000004
> [245864.699588] R10: ffffaaa545b23ed0 R11: 0000000000000001 R12: ffffffffc06b61a0
> [245864.699589] R13: ffffaaa540cffe60 R14: ffff9335c77fa3c0 R15: ffff9335cf51d7c0
> [245864.699590]  ? 0xffffffffc06b61a0
> [245864.699592]  ? 0xffffffffc06b67ec                                  <<<<<<<<
> [245864.699593]  ? 0xffffffffc06b67ec
> [245864.699594]  </NMI>
> 
> Note RIP: 0010:0xffffffffc06b67ec and 0xffffffffc06b67ec printed in above
> stack trace as absolute address. There is no easy way in case box crashes
> as we loose information on load address of specific module.

Mentioning this note makes little sense, just say how the above doesn't help you
easily identify the root cause of an address, and how this is just part
of consequences of stripping symbols. Because developers using
INSTALL_MOD_STRIP="--strip-unneeded"  know that they are trading off
debugging live crashes over reducing the kernel and module binary size.

> This changes the symbol decoding (in kernel/module.c) such that it can
> print offset from start of section (.text or .init.text) in case there
> is no good match with any symbol.

No, this is adding a new heuristic to help in cases where no good match
for a symbol was found, and typically this can be when symbols are
stripped.

> It will now decode address in such cases to [module]+ offset/size or
> [module __init]+offset/size depending on where the address lies (in
> core/.text or init/.init.text section of module).
> 
> One can use objdump/readelf/nm to find symbols with offset from .init.text
> and .text sections.
> 
> steps to reproduce the problem:
> -------------------------------
> 1. Add WARN_ON_ONCE(1) in module e.g. test_module.c
> 2. Build and strip the module using --strip-unneeded option
> 3. Load the module and check RIP in dmesg
> 
> tests done:
> -----------
> 1. Added WARN_ON_ONE(1) in functions of a module for testing
> -------------------------------------------------------------
> [  407.934085] CPU: 0 PID: 2956 Comm: insmod Tainted: G        W   E     5.16.0-rc5-next-20211220+ #2
> [  407.934087] Hardware name: innotek GmbH VirtualBox/VirtualBox, BIOS VirtualBox 12/01/2006
> [  407.934088] RIP: 0010:[module __init]+0x4/0x7 [test_module]
> [  407.934097] Code: Unable to access opcode bytes at RIP 0xffffffffc07edfda.
> [  407.934098] RSP: 0018:ffffb21440487c20 EFLAGS: 00010202
> [  407.934100] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
> [  407.934101] RDX: 0000000000000000 RSI: ffffffff9c38e5e1 RDI: 0000000000000001
> [  407.934102] RBP: ffffb21440487c28 R08: 0000000000000000 R09: ffffb21440487a20
> [  407.934103] R10: ffffb21440487a18 R11: ffffffff9c755248 R12: ffffffffc07ee007
> [  407.934104] R13: ffff92a0f1e260b0 R14: 0000000000000000 R15: 0000000000000000
> [  407.934105] FS:  00007f578ebc4400(0000) GS:ffff92a1c0e00000(0000) knlGS:0000000000000000
> [  407.934107] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [  407.934108] CR2: ffffffffc07edfda CR3: 00000000063ea006 CR4: 00000000000706f0
> [  407.934113] Call Trace:
> [  407.934114]  <TASK>
> [  407.934116]  ? init_module+0x55/0xff9 [test_module]
> ...
> [  407.934232] CPU: 0 PID: 2956 Comm: insmod Tainted: G        W   E     5.16.0-rc5-next-20211220+ #2
> [  407.934234] Hardware name: innotek GmbH VirtualBox/VirtualBox, BIOS VirtualBox 12/01/2006
> [  407.934242] RIP: 0010:[module]+0x4/0x7 [test_module]
> [  407.934248] Code: Unable to access opcode bytes at RIP 0xffffffffc07e1fda.
> [  407.934249] RSP: 0018:ffffb21440487c20 EFLAGS: 00010202
> [  407.934251] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
> [  407.934252] RDX: 0000000000000000 RSI: ffffffff9c38e5e1 RDI: 0000000000000001
> [  407.934253] RBP: ffffb21440487c28 R08: 0000000000000000 R09: ffffb21440487a20
> [  407.934254] R10: ffffb21440487a18 R11: ffffffff9c755248 R12: ffffffffc07ee007
> [  407.934255] R13: ffff92a0f1e260b0 R14: 0000000000000000 R15: 0000000000000000
> [  407.934256] FS:  00007f578ebc4400(0000) GS:ffff92a1c0e00000(0000) knlGS:0000000000000000
> [  407.934257] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [  407.934258] CR2: ffffffffc07e1fda CR3: 00000000063ea006 CR4: 00000000000706f0
> [  407.934260] Call Trace:
> [  407.934260]  <TASK>
> [  407.934261]  ? init_module+0x5a/0xff9 [test_module]
> 
> note that it is able to decode RIP to an offset from module start or
> init start now.
> 
> tested on linux->next (tag next-20211220)
> 
> Signed-off-by: Vimal Agrawal <vimal.agrawal@sophos.com>
> Acked-by: Nishit Shah <nishit.shah@sophos.com>
> Suggested-by: Dirk VanDerMerwe <Dirk.VanDerMerwe@Sophos.com>
> ---
>  kernel/module.c | 44 +++++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 39 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/module.c b/kernel/module.c
> index 24dab046e16c..635e631c92f1 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -4260,6 +4260,19 @@ static const char *kallsyms_symbol_name(struct mod_kallsyms *kallsyms, unsigned
>  	return kallsyms->strtab + kallsyms->symtab[symnum].st_name;
>  }
>  
> +/*
> + * Check if module address (addr) passed is as per init passed
> + * if init is true then check in init layout
> + * else check in core layout
> + */
> +bool match_address_layout(struct module *mod, unsigned long addr, bool init)
> +{
> +	if (init)
> +		return within_module_init(addr, mod);
> +	else
> +		return within_module_core(addr, mod);
> +}
> +
>  /*
>   * Given a module and address, find the corresponding symbol and return its name
>   * while providing its size and offset if needed.
> @@ -4270,14 +4283,22 @@ static const char *find_kallsyms_symbol(struct module *mod,
>  					unsigned long *offset)
>  {
>  	unsigned int i, best = 0;
> -	unsigned long nextval, bestval;
> +	unsigned long baseval, nextval, bestval;
>  	struct mod_kallsyms *kallsyms = rcu_dereference_sched(mod->kallsyms);
> +	char *module_base_name;
> +	bool is_init_address = false;
>  
>  	/* At worse, next value is at end of module */
> -	if (within_module_init(addr, mod))
> +	if (within_module_init(addr, mod)) {
> +		baseval = (unsigned long)mod->init_layout.base;
>  		nextval = (unsigned long)mod->init_layout.base+mod->init_layout.text_size;
> -	else
> +		module_base_name = "[module __init]";
> +		is_init_address = true;
> +	} else {
> +		baseval = (unsigned long)mod->core_layout.base;
>  		nextval = (unsigned long)mod->core_layout.base+mod->core_layout.text_size;
> +		module_base_name = "[module]";
> +	}
>  


To make all this readible and easier to follow please break down your
work into 3 patches:

1) You add the above branch and add baseval as you do now and you also
use it on the nextval assigmment. So something like this:

+             baseval = (unsigned long)mod->init_layout.base;                 
-             nextval = (unsigned long)mod->init_layout.base+mod->init_layout.text_size;
+             nextval = baseval + mod->init_layout.text_size;

You would do that for the core_layout branch too.

This would make only one single change to the code. That is just one
patch. And that patch introduces no functional changes. You would
clarify that on the commit log, to make it clear there is no possible
regression risks with it.

>  	bestval = kallsyms_symbol_value(&kallsyms->symtab[best]);
>  
> @@ -4300,7 +4321,8 @@ static const char *find_kallsyms_symbol(struct module *mod,
>  		    || is_arm_mapping_symbol(kallsyms_symbol_name(kallsyms, i)))
>  			continue;
>  
> -		if (thisval <= addr && thisval > bestval) {
> +		if ((thisval <= addr && thisval > bestval) &&
> +			(match_address_layout(mod, thisval, is_init_address))) {

OK your 2nd patch will be:

2) You add match_address_layout() and the is_init_address bool. This is
a fix found through code inspection. You can clarify that the issue has
only been found when --strip-unneeded is used, however, that the fix
could potentialy solve other false symbol. At this point in time no
known cases are known where the reported symbol returned is invalid,
but this sanity check makes sense even for cases where no striping is
used. You can also mention that you this fix is also being added to
be able to add a new heuristic later for cases where --strip-unneeded
is used.

>  			best = i;
>  			bestval = thisval;
>  		}
> @@ -4308,8 +4330,20 @@ static const char *find_kallsyms_symbol(struct module *mod,
>  			nextval = thisval;
>  	}
>  
> -	if (!best)
> +	if (!best) {
> +		if (is_module_text_address(addr)) {
> +			/*
> +			 * return MODULE base and offset if we could not find
> +			 * any best match for text address
> +			 */
> +			if (size)
> +				*size = nextval - baseval;
> +			if (offset)
> +				*offset = addr - baseval;
> +			return module_base_name;
> +		}
>  		return NULL;

And then patch 3 is this new change. Please test the config I sent you
though, I have not been able to reproduce your setup yet.

  Luis