Re: [PATCH, RESEND] ipc/shm: handle removed segments gracefully in shm_mmap()

From: Davidlohr Bueso
Date: Fri Nov 13 2015 - 00:31:50 EST

On Wed, 11 Nov 2015, Kirill A. Shutemov wrote:
And I had concern about your approach:

If I read it correctly, with the patch we would ignore locking
failure inside shm_open() and mmap will succeed in this case. So
the idea is to have shm_close() no-op and therefore symmetrical.

Both open and close are no-ops in the case the segment has been removed,
that's the symmetrical, and I'm not sure I follow -- we don't ignore locking
failure in shm_open _at all_. Just like your approach, all I do is return if
there's an error...

That's look fragile to me. We would silently miss some other
broken open/close pattern.

Such cases, if any, should be fixed and handled appropriately, not hide
it under the rung, methinks.

o My shm_check_vma_validity() also deals with IPC_RMID as we do the
ipc_valid_object() check.

Mine too:


Or I miss something?

Sorry, I meant ipc_obtain_object_idr, so EINVAL is also accounted for, we
the segment is already deleted and not only marked as such.

o We have a new WARN where necessary, instead of having one now is shm_open.

I'm not sure why you think that shm_close() which was never paired with
successful shm_open() doesn't deserve WARN().

o My no-ops explicitly pair.

As I said before, I don't think we should ignore locking error in
shm_open(). If we propagate the error back to caller shm_close() should
never happen, therefore no-op is unneeded in shm_close(): my patch trigger
WARN() there.

Yes, you WARN() in shm_close, but you still make it a no-op...

> ret = sfd->file->f_op->mmap(sfd->file, vma);
>- if (ret != 0)
>+ if (ret) {
>+ shm_close(vma);
> return ret;
>+ }

Hmm what's this shm_close() about?

Undo shp->shm_nattch++ in successful __shm_open().

Yeah that's just nasty.

I've got impression that I miss something important about how locking in
IPC/SHM works, but I cannot grasp what.. Hm?.

Could you be more specific? The only lock involved here is the ipc object lock,
if you haven't, you might want to refer to ipc/util.c which has a brief ipc
locking description.
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