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From: Ed Tomlinson <tomlins@cam.org>
Subject: Re: 2.5.31: modules don't work at all
To: linux-kernel@vger.kernel.org, Andrew Morton <akpm@zip.com.au>
Reply-To: tomlins@cam.org
Date: Tue, 20 Aug 2002 18:59:46 -0400
References: <200208120233.TAA16322@adam.yggdrasil.com> <200208120307.g7C37AuF000184@pool-141-150-241-241.delv.east.verizon.net> <3D574972.DD878928@zip.com.au>
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Andrew Morton wrote:

> Skip Ford wrote:
>> 
>> ...
>> >       I already know that the error that trips insmod occurs at
>> > in modules.c, line 831, when qm_symbols gets an error from
>> > copy_to_user():
>> >
>> >         for (; i < mod->nsyms ; ++i, ++s, vals += 2) {
>> >                 len = strlen(s->name)+1;
>> >                 if (len > bufsize)
>> >                         goto calc_space_needed;
>> >
>> > here------>     if (copy_to_user(strings, s->name, len)
>> >                     || __put_user(s->value, vals+0)
>> >                     || __put_user(space, vals+1))
>> >                         return -EFAULT;
>> >
>> >                 strings += len;
>> >                 bufsize -= len;
>> >                 space += len;
>> >         }
>> >
>> >       The values of strings and s->name are similar in 2.5.30+preempt
>> > (works) and 2.5.31+preempt (does not work).  strings is 0x08______, and
>> > s->name is 0xc0______.
>> 
>> If I back out this change to arch/i386/mm/fault.c then modules
>> successfully load.  I have no idea if backing it out causes other
>> problems though.
>> 
>> diff -Nru a/arch/i386/mm/fault.c b/arch/i386/mm/fault.c
>> --- a/arch/i386/mm/fault.c      Sat Aug 10 18:42:20 2002
>> +++ b/arch/i386/mm/fault.c      Sat Aug 10 18:42:20 2002
>> @@ -181,10 +181,10 @@
>>         info.si_code = SEGV_MAPERR;
>> 
>>         /*
>> -        * If we're in an interrupt or have no user
>> -        * context, we must not take the fault..
>> +        * If we're in an interrupt, have no user context or are running
>> in an
>> +        * atomic region then we must not take the fault..
>>          */
>> -       if (in_interrupt() || !mm)
>> +       if (preempt_count() || !mm)
>>                 goto no_context;
>> 
>>         down_read(&mm->mmap_sem);
>> 
> 
> Yes, that's the problem.   qm_symbols() is performing copy_to_user()
> inside lock_kernel() and that's an "atomic copy_to_user()" in 2.5.31.
> But only if preempt is selected.  The copy_to_user() doesn't work.
> 
> There's nothing illegal about copy_to_user() inside lock_kernel().
> 
> Linus, we can back out the preempt_count() test in there and
> perform the atomic copy_*_user via a current->flags bit, or
> we can do something else?

I am still seeing this problem with the todays bk current, which includes
the above 'fix'...

Turning off preempt now.

Ed Tomlinson
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