On 2019-01-08 20:36, Andrew Morton wrote:
On Tue, 8 Jan 2019 12:09:44 +0100 Roman Penyaev <rpenyaev@xxxxxxx>
wrote:
This patch repeats the original one from David S. Miller:
2dca6999eed5 ("mm, perf_event: Make vmalloc_user() align base kernel
virtual address to SHMLBA")
but for missed vmalloc_32_user() case, which also requires correct
alignment of virtual address on kernel side to avoid D-caches
aliases. A bit of copy-paste from original patch to recover in
memory of what is all about:
When a vmalloc'd area is mmap'd into userspace, some kind of
co-ordination is necessary for this to work on platforms with cpu
D-caches which can have aliases.
Otherwise kernel side writes won't be seen properly in userspace
and vice versa.
If the kernel side mapping and the user side one have the same
alignment, modulo SHMLBA, this can work as long as VM_SHARED is
shared of VMA and for all current users this is true. VM_SHARED
will force SHMLBA alignment of the user side mmap on platforms with
D-cache aliasing matters.
What are the user-visible runtime effects of this change?
In simple words: proper alignment avoids possible difference in data,
seen by different virtual mapings: userspace and kernel in our case.
I.e. userspace reads cache line A, kernel writes to cache line B.
Both cache lines correspond to the same physical memory (thus aliases).
So this should fix data corruption for archs with vivt and vipt caches,
e.g. armv6. Personally I've never worked with this archs, I just
spotted
the strange difference in code: for one case we do alignment, for
another
- not. I have a strong feeling that David simply missed
vmalloc_32_user()
case.
Is a -stable backport needed?
No, I do not think so. The only one user of vmalloc_32_user() is
virtual
frame buffer device drivers/video/fbdev/vfb.c, which has in the
description
"The main use of this frame buffer device is testing and debugging the
frame
buffer subsystem. Do NOT enable it for normal systems!".
And it seems to me that this vfb.c does not need 32bit addressable pages
(vmalloc_32_user() case), because it is virtual device and should not
care
about things like dma32 zones, etc. Probably is better to clean the
code
and switch vfb.c from vmalloc_32_user() to vmalloc_user() case and wipe
out
vmalloc_32_user() from vmalloc.c completely. But I'm not very much sure
that this is worth to do, that's so minor, so we can leave it as is.
--
Roman
