Hi,
James Hogan schrieb:
Hi,
I came across a problem in fb_write (drivers/video/fbmem.c) which doesn't make
any effort to avoid unaligned writes. It copies up to a page at a time from
userspace into a buffer, then fb_writel's as much as possible from the buffer
into the framebuffer, then fb_writeb's any remaining bytes. However on
architectures which don't perform unaligned writes this can cause a bus error
or silently fail when it tries to fb_writel to an unaligned framebuffer
position.
My question is how best should this be fixed? I know I could avoid writing
more than 3 bytes to an unaligned framebuffer offset, but the standard write
syscall should be able to handle this, and Documentation/unaligned-memory-
access.txt makes it clear that unaligned accesses are to be avoided and with
good reason.
Is it possible to just copy_from_user straight into the framebuffer, or should
I define an fb_memcpy to go with the other definitions around line 925 of
include/linux/fb.h, or something else?
Just doing copy_from_user straight into the framebuffer sounds like the right
thing to me but my knowledge is limited to x86. As far as I can see this would
make the functions (doing the same for fb_read can't hurt, can it?) in fbmem.c
very similar to those in fb_sys_fops.c. Most users of the later functions can be
easily eliminated as simply removing ".fb_{read,write} = fb_sys_{read,write}" is
sufficient, only xen-fbfront.c looks more difficult as it also refreshes the
screen so I'd suggest the following:
Merging fb_sys_fops.c in fbmem.c, adjusting the fb_{read,write} functions to use
the the fb_sys_{read,write} copying and removing all trivial uses of
fb_sys_{read,write}. As far as I can see this should solve your problem and
would be a cleanup at the same time.
But it would be good if someone who knows other archs (especially sparc as they
currently seem to perform some assembler that I can't decipher) could comment on
this topic.
Thanks,
Florian Tobias Schandinat
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