Re: Migration of kernel interfaces to seq_files breaks pread() consumers

[ebiederm@xmission.com (Eric W. Biederman)]

Alexey Dobriyan <adobriyan@gmail.com> writes:

> On Sat, Jan 24, 2009 at 06:19:24PM -0800, Andrew Morton wrote:
>> On Fri, 16 Jan 2009 23:51:35 -0800 (PST) Paul Turner <pjt@google.com> wrote:
>> 
>> > 
>> > (Specifically) Several interfaces under /proc have been migrated to use 
>> > seq_files.  This was previously observed to be a problem with VMware's 
>> > reading of /proc/uptime.  We're now running into the same problem on  
>> > /proc/<pid>/stat; we have many consumers performing preads on this 
>> > interface which break under new kernels.
>> > 
>> > Reverting these migrations presents other problems and doesn't scale with 
>> > everyones' pet dependencies over an abi that's been
>> > broken :(
>> 
>> We changed userspace-visible behaviour and broke real applications. 
>> This is a serious matter.  So serious in fact that your report has
>> languished without reply for a week.
>> 
>> Reverting those changes until we have a suitable reimplementation which
>> doesn't bust userspace is 100% justifiable.
>> 
>> In which kernel versions is this regression present?
>> 
>> What would a revert look like?  Big and ugly or small and simple?  Do
>> the original commits (which were they?) still revert OK?
>
> This is bug http://bugzilla.kernel.org/show_bug.cgi?id=11856
> Some of us think what to do here.
>
> Original patch not revertable as is.

Interesting.  This seems like a bug in seq_file plain and simple.
Userspace appears to be acting very reasonable in this case.

Why is there a notion that we have to differentiate between read
and pread in seq_file to fix this.  That doesn't make much sense.

Anyway here is an untested but logically correct patch which should fix
this issue, without nasty special casing of pread.

Eric