Re: [PATCH v15 05/17] arms64: untag user pointers passed to memory syscalls

Andrew Murray
2019-05-28T14:54:18Z

On Mon, May 27, 2019 at 03:37:20PM +0100, Catalin Marinas wrote:
> On Mon, May 06, 2019 at 06:30:51PM +0200, Andrey Konovalov wrote:
> > This patch is a part of a series that extends arm64 kernel ABI to allow to
> > pass tagged user pointers (with the top byte set to something else other
> > than 0x00) as syscall arguments.
> >
> > This patch allows tagged pointers to be passed to the following memory
> > syscalls: brk, get_mempolicy, madvise, mbind, mincore, mlock, mlock2,
> > mmap, mmap_pgoff, mprotect, mremap, msync, munlock, munmap,
> > remap_file_pages, shmat and shmdt.
> >
> > This is done by untagging pointers passed to these syscalls in the
> > prologues of their handlers.
> >
> > Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
>
> Actually, I don't think any of these wrappers get called (have you
> tested this patch?). Following commit 4378a7d4be30 ("arm64: implement
> syscall wrappers"), I think we have other macro names for overriding the
> sys_* ones.

What is the value in adding these wrappers?

The untagged_addr macro is defined for all in linux/mm.h and these patches
already use untagged_addr in generic code. Thus adding a few more
untagged_addr in the generic syscall handlers (which turn to a nop for most)
is surely better than adding wrappers?

Even if other architectures implement untagged_addr in the future it would
be more consistent if they untagged in the same places and thus not adding
these wrappers enforces that.

Thanks,

Andrew Murray

>
> --
> Catalin
>
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