shmat(2) returns page size aligned memory address

shmat(2) returns page size aligned memory address

[Yubin Ruan]

Hi Michael,
At the current man page for shmat(2)[1], there is no mentioning
whether the returned memory address of shmat(2) will be page size
aligned or not. As that is quite important to many applications(e.g.,
those that use locks heavily and would like to avoid some locks by
some atomic guarantees provided by the CPU), it would be great to
specify that for Linux.

I walked down the current implementation of shmat(2) in the latest
kernel src and found that shmat(2) does return a page size aligned
memory address:

SYSCALL_DEFINE3(shmat, int, shmid, char __user *, shmaddr, int, shmflg)
 -> do_shmat(...)
 -> do_mmap_pgoff(...)
 -> do_mmap(...)
 -> get_unmapped_area(...)
 -> get_area(...) -> offset_in_page(addr)

there is a `offset_in_page(addr)' assertion at the end and if that is
true a -EINVAL would be returned, by which we can be sure that
shmat(2) will return a page size aligned memory address on success[2].

I will create a patch later if that is acceptable.

Thanks,
Yubin

[1]: http://man7.org/linux/man-pages/man2/shmat.2.html
[2]: there is also a `offset_in_page(2)' in get_unmapped_area(...),
but that doesn't lead to -EINVAL...I am not sure whether the logic of
that code is right.
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Re: shmat(2) returns page size aligned memory address

[Yubin Ruan]

Cc linux-mm

2017-10-08 23:37 GMT+08:00 Yubin Ruan <ablacktshirt@gmail.com>:
> Hi Michael,
> At the current man page for shmat(2)[1], there is no mentioning
> whether the returned memory address of shmat(2) will be page size
> aligned or not. As that is quite important to many applications(e.g.,
> those that use locks heavily and would like to avoid some locks by
> some atomic guarantees provided by the CPU), it would be great to
> specify that for Linux.
>
> I walked down the current implementation of shmat(2) in the latest
> kernel src and found that shmat(2) does return a page size aligned
> memory address:
>
> SYSCALL_DEFINE3(shmat, int, shmid, char __user *, shmaddr, int, shmflg)
>  -> do_shmat(...)
>  -> do_mmap_pgoff(...)
>  -> do_mmap(...)
>  -> get_unmapped_area(...)
>  -> get_area(...) -> offset_in_page(addr)
>
> there is a `offset_in_page(addr)' assertion at the end and if that is
> true a -EINVAL would be returned, by which we can be sure that
> shmat(2) will return a page size aligned memory address on success[2].
>
> I will create a patch later if that is acceptable.
>
> Thanks,
> Yubin
>
> [1]: http://man7.org/linux/man-pages/man2/shmat.2.html
> [2]: there is also a `offset_in_page(2)' in get_unmapped_area(...),
> but that doesn't lead to -EINVAL...I am not sure whether the logic of
> that code is right.

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[PATCH] shmat(2) returns page size aligned memory address

[Yubin Ruan]

On Sun, Oct 08, 2017 at 11:37:05PM +0800, Yubin Ruan wrote:
> Hi Michael,
> At the current man page for shmat(2)[1], there is no mentioning
> whether the returned memory address of shmat(2) will be page size
> aligned or not. As that is quite important to many applications(e.g.,
> those that use locks heavily and would like to avoid some locks by
> some atomic guarantees provided by the CPU), it would be great to
> specify that for Linux.
> 
> I walked down the current implementation of shmat(2) in the latest
> kernel src and found that shmat(2) does return a page size aligned
> memory address:
> 
> SYSCALL_DEFINE3(shmat, int, shmid, char __user *, shmaddr, int, shmflg)
>  -> do_shmat(...)
>  -> do_mmap_pgoff(...)
>  -> do_mmap(...)
>  -> get_unmapped_area(...)
>  -> get_area(...) -> offset_in_page(addr)
> 
> there is a `offset_in_page(addr)' assertion at the end and if that is
> true a -EINVAL would be returned, by which we can be sure that
> shmat(2) will return a page size aligned memory address on success[2].
> 
> I will create a patch later if that is acceptable.
> 
> Thanks,
> Yubin
> 
> [1]: http://man7.org/linux/man-pages/man2/shmat.2.html
> [2]: there is also a `offset_in_page(2)' in get_unmapped_area(...),
> but that doesn't lead to -EINVAL...I am not sure whether the logic of
> that code is right.

add the page-alignment attribute of the return address of shmat(2)
---

diff --git a/man2/shmop.2 b/man2/shmop.2
index 849529f..b8d7595 100644
--- a/man2/shmop.2
+++ b/man2/shmop.2
@@ -63,7 +63,7 @@ with one of the following criteria:
 If
 .I shmaddr
 is NULL,
-the system chooses a suitable (unused) address at which to attach
+the system chooses a suitable (unused) page-aligned address to attach
 the segment.
 .IP *
 If

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Re: [PATCH] shmat(2) returns page size aligned memory address

[Michael Kerrisk (man-opages)]

Thanks, Yubin. Patch applied.

Cheers,

Michael


On 10/09/2017 04:27 PM, Yubin Ruan wrote:
> On Sun, Oct 08, 2017 at 11:37:05PM +0800, Yubin Ruan wrote:
>> Hi Michael,
>> At the current man page for shmat(2)[1], there is no mentioning
>> whether the returned memory address of shmat(2) will be page size
>> aligned or not. As that is quite important to many applications(e.g.,
>> those that use locks heavily and would like to avoid some locks by
>> some atomic guarantees provided by the CPU), it would be great to
>> specify that for Linux.
>>
>> I walked down the current implementation of shmat(2) in the latest
>> kernel src and found that shmat(2) does return a page size aligned
>> memory address:
>>
>> SYSCALL_DEFINE3(shmat, int, shmid, char __user *, shmaddr, int, shmflg)
>>   -> do_shmat(...)
>>   -> do_mmap_pgoff(...)
>>   -> do_mmap(...)
>>   -> get_unmapped_area(...)
>>   -> get_area(...) -> offset_in_page(addr)
>>
>> there is a `offset_in_page(addr)' assertion at the end and if that is
>> true a -EINVAL would be returned, by which we can be sure that
>> shmat(2) will return a page size aligned memory address on success[2].
>>
>> I will create a patch later if that is acceptable.
>>
>> Thanks,
>> Yubin
>>
>> [1]: http://man7.org/linux/man-pages/man2/shmat.2.html
>> [2]: there is also a `offset_in_page(2)' in get_unmapped_area(...),
>> but that doesn't lead to -EINVAL...I am not sure whether the logic of
>> that code is right.
> 
> add the page-alignment attribute of the return address of shmat(2)
> ---
> 
> diff --git a/man2/shmop.2 b/man2/shmop.2
> index 849529f..b8d7595 100644
> --- a/man2/shmop.2
> +++ b/man2/shmop.2
> @@ -63,7 +63,7 @@ with one of the following criteria:
>   If
>   .I shmaddr
>   is NULL,
> -the system chooses a suitable (unused) address at which to attach
> +the system chooses a suitable (unused) page-aligned address to attach
>   the segment.
>   .IP *
>   If
> 
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