linux-kernel - Re: [PATCH 14/14] powerpc: remove address space overrides using set

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Date:   Thu, 3 Sep 2020 17:56:44 +0200
From:   Christoph Hellwig <hch@....de>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Christoph Hellwig <hch@....de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Michael Ellerman <mpe@...erman.id.au>, x86@...nel.org,
        linux-arch@...r.kernel.org, Kees Cook <keescook@...omium.org>,
        linux-kernel@...r.kernel.org, Luis Chamberlain <mcgrof@...nel.org>,
        linux-fsdevel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Alexey Dobriyan <adobriyan@...il.com>
Subject: Re: [PATCH 14/14] powerpc: remove address space overrides using
 set_fs()

On Thu, Sep 03, 2020 at 05:49:09PM +0200, Christoph Hellwig wrote:
> On Thu, Sep 03, 2020 at 05:43:25PM +0200, Christophe Leroy wrote:
> >
> >
> > Le 03/09/2020 à 16:22, Christoph Hellwig a écrit :
> >> Stop providing the possibility to override the address space using
> >> set_fs() now that there is no need for that any more.
> >>
> >> Signed-off-by: Christoph Hellwig <hch@....de>
> >> ---
> >
> >
> >>   -static inline int __access_ok(unsigned long addr, unsigned long size,
> >> -			mm_segment_t seg)
> >> +static inline bool __access_ok(unsigned long addr, unsigned long size)
> >>   {
> >> -	if (addr > seg.seg)
> >> -		return 0;
> >> -	return (size == 0 || size - 1 <= seg.seg - addr);
> >> +	if (addr >= TASK_SIZE_MAX)
> >> +		return false;
> >> +	return size == 0 || size <= TASK_SIZE_MAX - addr;
> >>   }
> >
> > You don't need to test size == 0 anymore. It used to be necessary because 
> > of the 'size - 1', as size is unsigned.
> >
> > Now you can directly do
> >
> > 	return size <= TASK_SIZE_MAX - addr;
> >
> > If size is 0, this will always be true (because you already know that addr 
> > is not >= TASK_SIZE_MAX
> 
> True.  What do you think of Linus' comment about always using the
> ppc32 version on ppc64 as well with this?

i.e. something like this folded in:

diff --git a/arch/powerpc/include/asm/uaccess.h b/arch/powerpc/include/asm/uaccess.h
index 5363f7fc6dd06c..be070254e50943 100644
--- a/arch/powerpc/include/asm/uaccess.h
+++ b/arch/powerpc/include/asm/uaccess.h
@@ -11,26 +11,14 @@
 #ifdef __powerpc64__
 /* We use TASK_SIZE_USER64 as TASK_SIZE is not constant */
 #define TASK_SIZE_MAX		TASK_SIZE_USER64
-
-/*
- * This check is sufficient because there is a large enough gap between user
- * addresses and the kernel addresses.
- */
-static inline bool __access_ok(unsigned long addr, unsigned long size)
-{
-	return addr < TASK_SIZE_MAX && size < TASK_SIZE_MAX;
-}
-
 #else
 #define TASK_SIZE_MAX		TASK_SIZE
+#endif
 
 static inline bool __access_ok(unsigned long addr, unsigned long size)
 {
-	if (addr >= TASK_SIZE_MAX)
-		return false;
-	return size == 0 || size <= TASK_SIZE_MAX - addr;
+	return addr < TASK_SIZE_MAX && size <= TASK_SIZE_MAX - addr;
 }
-#endif /* __powerpc64__ */
 
 #define access_ok(addr, size)		\
 	(__chk_user_ptr(addr),		\