Dmitry Vyukov <dvyukov@xxxxxxxxxx> writes:
> On Thu, Dec 3, 2015 at 6:02 PM, Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
>> On Thu, Dec 3, 2015 at 7:55 AM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
>>> On Thu, Dec 3, 2015 at 3:48 PM, Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
>>>>>
>>>>> No, I don't. But pr_debug always computes its arguments. See no_printk
>>>>> in printk.h. So this use-after-free happens for all users.
>>>>
>>>> Hmm.
>>>>
>>>> pr_debug() should be a nop unless either DEBUG or
>>>> CONFIG_DYNAMIC_DEBUG are set
>>>>
>>>> On our production kernels, pr_debug() is a nop.
>>>>
>>>> Can you double check ? Thanks !
>>>
>>>
>>> Why should it be nop? no_printk thing in printk.h pretty much
>>> explicitly makes it not a nop...
Because it was until commit 5264f2f75d8. It also violates my reading of
the following from printk.h:
* All of these will print unconditionally, although note that pr_debug()
* and other debug macros are compiled out unless either DEBUG is defined
* or CONFIG_DYNAMIC_DEBUG is set.
>>>
>>> Double-checked: debug_post_sfx leads to some generated code:
>>>
>>> debug_post_sfx();
>>> ffffffff8229f256: 48 8b 85 58 fe ff ff mov -0x1a8(%rbp),%rax
>>> ffffffff8229f25d: 48 85 c0 test %rax,%rax
>>> ffffffff8229f260: 74 24 je
>>> ffffffff8229f286 <sctp_do_sm+0x176>
>>> ffffffff8229f262: 8b b0 a8 00 00 00 mov 0xa8(%rax),%esi
>>> ffffffff8229f268: 48 8b 85 60 fe ff ff mov -0x1a0(%rbp),%rax
>>> ffffffff8229f26f: 44 89 85 74 fe ff ff mov %r8d,-0x18c(%rbp)
>>> ffffffff8229f276: 48 8b 78 20 mov 0x20(%rax),%rdi
>>> ffffffff8229f27a: e8 71 28 01 00 callq
>>> ffffffff822b1af0 <sctp_id2assoc>
>>> ffffffff8229f27f: 44 8b 85 74 fe ff ff mov -0x18c(%rbp),%r8d
>>>
>>> return error;
>>> }
>>> ffffffff8229f286: 48 81 c4 a0 01 00 00 add $0x1a0,%rsp
>>> ffffffff8229f28d: 44 89 c0 mov %r8d,%eax
>>> ffffffff8229f290: 5b pop %rbx
>>> ffffffff8229f291: 41 5c pop %r12
>>> ffffffff8229f293: 41 5d pop %r13
>>> ffffffff8229f295: 41 5e pop %r14
>>> ffffffff8229f297: 41 5f pop %r15
>>> ffffffff8229f299: 5d pop %rbp
>>> ffffffff8229f29a: c3 retq
>>
>> This is a serious concern, because we let in the past lot of patches
>> converting traditional
+1
>> #ifdef DEBUG
>> # define some_hand_coded_ugly_debug() printk( ...._
>> #else
>> # define some_hand_coded_ugly_debug()
>> #endif
>>
>> On the premise pr_debug() would be a nop.
>>
>> It seems it is not always the case. This is a very serious problem.
+1
>> We probably have hundred of potential bugs, because few people
>> actually make sure all debugging stuff is correct,
>> like comments can be wrong because they are not updated properly as
>> time flies.
>>
>> It is definitely a nop for many cases.
>>
>> +void eric_test_pr_debug(struct sock *sk)
>> +{
>> + if (atomic_read(&sk->sk_omem_alloc))
>> + pr_debug("%s: optmem leakage for sock %p\n",
>> + __func__, sk);
>> +}
>>
>> ->
>>
>> 0000000000004740 <eric_test_pr_debug>:
>> 4740: e8 00 00 00 00 callq 4745 <eric_test_pr_debug+0x5>
>> 4741: R_X86_64_PC32 __fentry__-0x4
>> 4745: 55 push %rbp
>> 4746: 8b 87 24 01 00 00 mov 0x124(%rdi),%eax //
>> atomic_read() but nothing follows
>> 474c: 48 89 e5 mov %rsp,%rbp
>> 474f: 5d pop %rbp
>> 4750: c3 retq
>
>
>
> I would expect that it is nop when argument evaluation does not have
> side-effects. For example, for a load of a variable compiler will most
> likely elide it (though, it does not have to elide it, because the
> load is spelled in the code, so it can also legally emit the load and
> doesn't use the result).
> But if argument computation has side-effect (or compiler can't prove
> otherwise), it must emit code. It must emit code for function calls
> when the function is defined in a different translation unit, and for
> volatile accesses (most likely including atomic accesses), etc
This isn't 100% true. As you state, in order to reach the return 0, all
side effects must be evaluated. Load generally does not have side
effects, so it can be safely elided, but function() must be emitted.
However, that is _not_ required to get the desired warning emission on a
printf argument function, see http://pastebin.com/UHuaydkj for an
example.
I think that as a minimum, the following patch should be evaluted, but am
unsure to whom I should submit it (after I test):
diff --git a/include/linux/printk.h b/include/linux/printk.h
index 9729565..cd24d2d 100644
--- a/include/linux/printk.h
+++ b/include/linux/printk.h
@@ -286,7 +286,7 @@ extern asmlinkage void dump_stack(void) __cold;
printk(KERN_DEBUG pr_fmt(fmt), ##__VA_ARGS__)
#else
#define pr_debug(fmt, ...) \
- no_printk(KERN_DEBUG pr_fmt(fmt), ##__VA_ARGS__)
+ ({ if(0) printk(KERN_DEBUG pr_fmt(fmt), ##__VA_ARGS__); 0;})
#endif
/*
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