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Date:   Tue, 1 Sep 2020 15:56:30 -0400
From:   Prarit Bhargava <prarit@...hat.com>
To:     Lucas De Marchi <lucas.de.marchi@...il.com>,
        Qu Wenruo <wqu@...e.com>
Cc:     lkml <linux-kernel@...r.kernel.org>,
        linux-modules <linux-modules@...r.kernel.org>,
        Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH] module: Add more error message for failed kernel module
 loading



On 9/1/20 2:50 PM, Lucas De Marchi wrote:
> On Sat, Aug 29, 2020 at 4:15 AM Qu Wenruo <wqu@...e.com> wrote:
>>
>> When kernel module loading failed, user space only get one of the
>> following error messages:
>> - -ENOEXEC
>>   This is the most confusing one. From corrupted ELF header to bad
>>   WRITE|EXEC flags check introduced by in module_enforce_rwx_sections()
>>   all returns this error number.
>>
>> - -EPERM
>>   This is for blacklisted modules. But mod doesn't do extra explain
>>   on this error either.
>>
>> - -ENOMEM
>>   The only error which needs no explain.
>>
>> This means, if a user got "Exec format error" from modprobe, it provides
>> no meaningful way for the user to debug, and will take extra time
>> communicating to get extra info.
>>
>> So this patch will add extra error messages for -ENOEXEC and -EPERM
>> errors, allowing user to do better debugging and reporting.
>>
>> Signed-off-by: Qu Wenruo <wqu@...e.com>
>> ---
>>  kernel/module.c | 11 +++++++++--
>>  1 file changed, 9 insertions(+), 2 deletions(-)
>>
>> diff --git a/kernel/module.c b/kernel/module.c
>> index 1c5cff34d9f2..9f748c6eeb48 100644
>> --- a/kernel/module.c
>> +++ b/kernel/module.c
>> @@ -2096,8 +2096,12 @@ static int module_enforce_rwx_sections(Elf_Ehdr *hdr, Elf_Shdr *sechdrs,
>>         int i;
>>
>>         for (i = 0; i < hdr->e_shnum; i++) {
>> -               if ((sechdrs[i].sh_flags & shf_wx) == shf_wx)
>> +               if ((sechdrs[i].sh_flags & shf_wx) == shf_wx) {
>> +                       pr_err(
>> +                       "Module %s section %d has invalid WRITE|EXEC flags\n",
>> +                               mod->name, i);
>>                         return -ENOEXEC;
>> +               }
>>         }
>>
>>         return 0;
>> @@ -3825,8 +3829,10 @@ static int load_module(struct load_info *info, const char __user *uargs,
>>         char *after_dashes;
>>
>>         err = elf_header_check(info);
>> -       if (err)
>> +       if (err) {
>> +               pr_err("Module has invalid ELF header\n");
>>                 goto free_copy;
>> +       }
>>
>>         err = setup_load_info(info, flags);
>>         if (err)
>> @@ -3834,6 +3840,7 @@ static int load_module(struct load_info *info, const char __user *uargs,
>>
>>         if (blacklisted(info->name)) {
>>                 err = -EPERM;
>> +               pr_err("Module %s is blacklisted\n", info->name);
> 
> I wonder why would anyone actually add the blacklist to the command
> line like this and have no
> way to revert that back. This was introduced in

Debug.  Debug.  Debug. ;)  The parameter was added to debug broken installations
and kernel boots.

> be7de5f91fdc modules: Add kernel parameter to blacklist modules
> as a way to overcome broken initrd generation afaics.

Not the generation of the initramfs, but blocking a module loaded during the
boot.  The installation may have failed and there's no easy way to then debug
what module was responsible for the failure.

 Either kernel
> command line (using modprobe.blacklist)
> or /etc/modprobe.d options are honoured by libkmod and allow a
> sufficiently privileged user to bypass it.
> 

Both of those options only work if the filesystem is mounted IIRC.  It could be
the case that modprobe.blacklist now works earlier in the boot, however, I've
never used it because module_blacklist is the biggest and best hammer that I can
use to get through a broken installation or boot.

In any case you're incorrectly assuming that the system has a filesystem on it.
That's not necessarily the case as I'll explain below.

> +Rusty, +Prarit: is there anything this module parameter is covering
> that I'm missing?

This is the situation I have repeatedly come across :  A system at a remote site
will not boot any flavor of Linux.  Since the system would not install the only
way to debug was to provide install images to workaround a module load failure.
As you can imagine that is a time consuming process as well as a bad end user
experience.

I got so sick of it that I wrote the code above (well similar to it anyway --
thanks for the review Randy ;)) to add the module_blacklist parameter to make
debugging an uninstallable "bricked" system easier.

It's come in handy in some unexpected ways.  We've had situations where modules
have loaded and corrupted memory and blacklisting them revealed that the modules
were responsible for the memory corruption.

P.

> 
> For the changes here,
> 
> Reviewed-by: Lucas De Marchi <lucas.demarchi@...el.com>
> 
> thanks
> Lucas De Marchi
> 
>>                 goto free_copy;
>>         }
>>
>> --
>> 2.27.0
>>
> 

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