Re: [PATCH] parisc: fix re-mmap() of shared file to same fixed address

[Helge Deller <deller@gmx.de>]

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Hi Mike,

On 11/25/2013 05:41 AM, Mike Frysinger wrote:
> On Wednesday 20 November 2013 17:07:42 Helge Deller wrote:
>> locale-gen from glibc showed a strange problem on parisc:
> 
> pedantic: locale-gen is from distros while glibc itself provides localedef.

Ok.

>> mmap2(NULL, 536870912, PROT_NONE, MAP_SHARED, 3, 0) = 0x42a54000
>> mmap2(0x42a54000, 103860, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_FIXED, 3, 0)
>> = -1 EINVAL (Invalid argument)
>>
>> Basically it was just trying to re-mmap() a file at the same address
>> which it was given by a previous mmap() call. But this remapping failed
>> with EINVAL.
>>
>> The problem is, that when MAP_FIXED and MAP_SHARED flags were used, we
>> didn't included the mapping-based offset when we verified the alignment of
>> the given fixed address against the offset which we calculated it in the
>> previous call.
> 
> hmm, which version of glibc were you trying ?  we landed this change recently:
> https://sourceware.org/git/?p=glibc.git;a=commitdiff_plain;h=17db6e8d6b12f55e312fcab46faf5d332c806fb6
> 
> it's part of glibc-2.18.  we know earlier versions of localedef failed on 
> systems like parisc, although it seemed to really only creep up on 64bit 
> kernels reliably.

It's debian's glibc 2.17-96

> based on your description, it sounds like independent bugs, but it also seems 
> a little coincidental ...

It's at least related.
As long as in the second mmap(ptr,...) the ptr value given is the same as the ptr value
returned by a prior call to mmap(NULL,...) it works.
It's hard to read from the diff if this is the case with the new glibc patch...

Anyway, I think the main thing one should know is, that a mmap(NULL,..) call will in most
cases *not* return a value which is a multiple of SHMLBA. But the offset (ptr%SHMLBA) 
will stay constant as long as the same file handle is used.

Helge
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