Re: [PATCH v3] kernel/module.c: heuristic enhancement in case symbols are missing e.g. when INSTALL_MOD_STRIP= "--strip-unneeded" is used

From: Luis Chamberlain
Date: Wed Feb 02 2022 - 15:20:16 EST

The subject should only be about 70-75 characters. You're going
way overboard here..

Try:

modules: add heuristic when stripping unneeded symbols

Then please describe in your commit log that on OpenWrt the
INSTALL_MOD_STRIP="--strip-unneeded" at kernel/module install
time, and so modules are stripped of unneeded symbols.

Your commit log currently does not give us the idea that this
is in fact a useful feature to consider, mentioning that this
is in fact used in OpenWrt tries to help us get a better idea
of a possible user of it and a potential benefit.

On Mon, Jan 17, 2022 at 12:24:52PM +0530, Vimal Agrawal wrote:
> If kernel modules are stripped off symbols for some reason (e.g. built by
> using strip --strip-unneeded option) 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)
> e.g.
>
> [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. 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.
>
> 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)

Sorry but for some reason this is not working on my end, but then
again I'm not able to get the stripped out results you see either.
So it could be I just have too many debugging options enabled
that makes INSTALL_MOD_STRIP="--strip-unneeded" not really do
much. I don't know if that is possible but I will have to try
to reduce my build options to test.

> Signed-off-by: Vimal Agrawal <vimal.agrawal@xxxxxxxxxx>
> ---
> kernel/module.c | 26 +++++++++++++++++++++++---
> 1 file changed, 23 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/module.c b/kernel/module.c
> index 24dab046e16c..4de15c06e760 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -4270,14 +4270,21 @@ 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;
>
> /* 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]";
> +
> + } else {
> + baseval = (unsigned long)mod->core_layout.base;
> nextval = (unsigned long)mod->core_layout.base+mod->core_layout.text_size;
> + module_base_name = "[module]";
> + }
>
> bestval = kallsyms_symbol_value(&kallsyms->symtab[best]);
>
> @@ -4308,6 +4315,19 @@ static const char *find_kallsyms_symbol(struct module *mod,
> nextval = thisval;
> }
>
> + if ((is_module_text_address(addr) &&
> + (bestval < baseval || bestval > nextval))) {
> + /*
> + * 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;
> + }
> +

I don't like this code to go before the check for !best. You did not
have this in your v1 or v2 patch. Why did you change it?

Luis

> if (!best)
> return NULL;
>
> --
> 2.32.0
>