Re: [PATCH] misc: sgi-xp: Properly initialize buf in xpc_get_rsvd_page_pa
From: Greg Kroah-Hartman
Date: Fri May 24 2019 - 12:03:22 EST
On Fri, May 24, 2019 at 09:40:47AM +0200, Arnd Bergmann wrote:
> On Thu, May 23, 2019 at 8:05 PM 'Nick Desaulniers' via Clang Built
> Linux <clang-built-linux@xxxxxxxxxxxxxxxx> wrote:
> > On Thu, May 23, 2019 at 9:20 AM Nathan Chancellor
> > <natechancellor@xxxxxxxxx> wrote:
> > >
> > > Clang warns:
> > >
> > > drivers/misc/sgi-xp/xpc_partition.c:73:14: warning: variable 'buf' is
> > > uninitialized when used within its own initialization [-Wuninitialized]
> > > void *buf = buf;
> > > ~~~ ^~~
> > > 1 warning generated.
> > >
> > > Initialize it to NULL, which is more deterministic.
> > >
> > > Fixes: 279290294662 ("[IA64-SGI] cleanup the way XPC locates the reserved page")
> > > Link: https://github.com/ClangBuiltLinux/linux/issues/466
> > > Suggested-by: Stephen Hines <srhines@xxxxxxxxxx>
> > > Signed-off-by: Nathan Chancellor <natechancellor@xxxxxxxxx>
> > From https://github.com/ClangBuiltLinux/linux/issues/466#issuecomment-488781917
> > I tried to follow the rabbit hole, but eventually these void* get
> > converted to u64's and passed along to function that I have no idea
> > whether they handle the value `(u64)(void*)0` or not. Either way,
> > they definitely don't handle uninitialized values/UB.
> > I was going to cc Robin who's already cc'ed, but looks like this code
> > was last touched 7-10 years ago. + Tony and Fenghua for ia64 since
> > sn_partition_reserved_page_pa is defined in
> > arch/ia64/include/asm/sn/sn_sal.h.
> > In absence of consensus, I'll prefer NULL to uninitialized.
> > Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> > Thanks Nathan for following up on this.
> I also had to take a look, and I think I understand what's going on,
> and interestingly, the code is correct, both before and after your patch.
> It's described in this comment:
> * Returns the physical address of the partition's reserved page through
> * an iterative number of calls.
> * On first call, 'cookie' and 'len' should be set to 0, and 'addr'
> * set to the nasid of the partition whose reserved page's address is
> * being sought.
> * On subsequent calls, pass the values, that were passed back on the
> * previous call.
> * While the return status equals SALRET_MORE_PASSES, keep calling
> * this function after first copying 'len' bytes starting at 'addr'
> * into 'buf'. Once the return status equals SALRET_OK, 'addr' will
> * be the physical address of the partition's reserved page. If the
> * return status equals neither of these, an error as occurred.
> static inline s64
> sn_partition_reserved_page_pa(u64 buf, u64 *cookie, u64 *addr, u64 *len)
> so *len is set to zero on the first call and tells the bios how many bytes
> are accessible at 'buf', and it does get updated by the BIOS to tell
> us how many bytes it needs, and then we allocate that and try again.
> With that explanation added,
> Reviewed-by: Arnd Bergmann <arnd@xxxxxxxx>
Nathan, can you add this to the changelog comment and add the reviewed
by lines and resend it so I can queue it up?