Re: [PATCH] bio: return EINTR if copying to user space got interrupted

[Hannes Reinecke <hare@suse.de>]

On 02/12/2016 05:05 PM, Douglas Gilbert wrote:
> On 16-02-12 03:39 AM, Hannes Reinecke wrote:
>> Commit 35dc248383bbab0a7203fca4d722875bc81ef091 introduced a check for
>> current->mm to see if we have a user space context and only copies data
>> if we do. Now if an IO gets interrupted by a signal data isn't copied
>> into user space any more (as we don't have a user space context) but
>> user space isn't notified about it.
>>
>> This patch modifies the behaviour to return -EINTR from bio_uncopy_user()
>> to notify userland that a signal has interrupted the syscall, otherwise
>> it could lead to a situation where the caller may get a buffer with
>> no data returned.
>
> Interesting, the "f091" commit has been in the kernel since 2013
> hence your reference to v.3.11 . I always had the feeling that
> handling signals that interrupted SG_IO calls was skating on thin
> ice.

Yeah; that bug was really annoying, as occasionally receiving no data 
whatsoever without any indication makes it really, really hard to debug.
Kudos go to Johannes and Ewan for pointing to the offending function.

> Hence in ddpt (but not sg_dd nor sgp_dd) the code masks out
> all signals (that it can) during the SG_IO calls then opens a
> signal window briefly after a SG_IO ioctl has finished and before
> the next one starts. This approach used by ddpt is borrowed from
> dd (in coreutils) which masks signals during its read() and
> write() calls.
>
> Any idea how accurate resid is in this scenario?
>
Bah. F*sk knows.
That takes far more POSIX knowledge than I have.
I would suspect that by masking out signals they'll be marked as pending 
for the process, and will be delivered once you unmask them.
Or dropped, depending.
In either case, once they are blocked out the kernel part shouldn't 
receive a signal, and hence we should be able to receive the data properly.
But as noted I'm not a POSIX expert.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
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