[tip:x86/asm] x86: Replace assembly access_ok() with a C variant
Commit-ID: c5fe5d80680e2949ffe102180f5fc6cefc0d145f
Gitweb: http://git.kernel.org/tip/c5fe5d80680e2949ffe102180f5fc6cefc0d145f
Author: Linus Torvalds
AuthorDate: Fri, 27 Dec 2013 15:30:58 -0800
Committer: H. Peter Anvin
CommitDate: Fri, 27 Dec 2013 16:58:17 -0800
x86: Replace assembly access_ok() with a C variant
It turns out that the assembly variant doesn't actually produce that
good code, presumably partly because it creates a long dependency
chain with no scheduling, and partly because we cannot get a flags
result out of gcc (which could be fixed with asm goto, but it turns
out not to be worth it.)
The C code allows gcc to schedule and generate multiple (easily
predictable) branches, and as a side benefit we can really optimize
the case where the size is constant.
Link:
http://lkml.kernel.org/r/CA%2B55aFzPBdbfKovMT8Edr4SmE2_=%2bokjfac9xw2awegogtk...@mail.gmail.com
Signed-off-by: H. Peter Anvin
---
arch/x86/include/asm/uaccess.h | 28 +---
1 file changed, 17 insertions(+), 11 deletions(-)
diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index 8ec57c0..84ecf1d 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -40,22 +40,28 @@
/*
* Test whether a block of memory is a valid user space address.
* Returns 0 if the range is valid, nonzero otherwise.
- *
- * This is equivalent to the following test:
- * (u33)addr + (u33)size > (u33)current->addr_limit.seg (u65 for x86_64)
- *
- * This needs 33-bit (65-bit for x86_64) arithmetic. We have a carry...
*/
+static inline int __chk_range_not_ok(unsigned long addr, unsigned long size,
unsigned long limit)
+{
+ /*
+* If we have used "sizeof()" for the size,
+* we know it won't overflow the limit (but
+* it might overflow the 'addr', so it's
+* important to subtract the size from the
+* limit, not add it to the address).
+*/
+ if (__builtin_constant_p(size))
+ return addr > limit - size;
+
+ /* Arbitrary sizes? Be careful about overflow */
+ addr += size;
+ return (addr < size) || (addr > limit);
+}
#define __range_not_ok(addr, size, limit) \
({ \
- unsigned long flag, roksum; \
__chk_user_ptr(addr); \
- asm("add %3,%1 ; sbb %0,%0 ; cmp %1,%4 ; sbb $0,%0" \
- : "=" (flag), "=r" (roksum) \
- : "1" (addr), "g" ((long)(size)), \
- "rm" (limit));\
- flag; \
+ __chk_range_not_ok((unsigned long __force)(addr), size, limit); \
})
/**
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[tip:x86/asm] x86: Replace assembly access_ok() with a C variant
Commit-ID: c5fe5d80680e2949ffe102180f5fc6cefc0d145f
Gitweb: http://git.kernel.org/tip/c5fe5d80680e2949ffe102180f5fc6cefc0d145f
Author: Linus Torvalds torva...@linux-foundation.org
AuthorDate: Fri, 27 Dec 2013 15:30:58 -0800
Committer: H. Peter Anvin h...@zytor.com
CommitDate: Fri, 27 Dec 2013 16:58:17 -0800
x86: Replace assembly access_ok() with a C variant
It turns out that the assembly variant doesn't actually produce that
good code, presumably partly because it creates a long dependency
chain with no scheduling, and partly because we cannot get a flags
result out of gcc (which could be fixed with asm goto, but it turns
out not to be worth it.)
The C code allows gcc to schedule and generate multiple (easily
predictable) branches, and as a side benefit we can really optimize
the case where the size is constant.
Link:
http://lkml.kernel.org/r/CA%2B55aFzPBdbfKovMT8Edr4SmE2_=%2bokjfac9xw2awegogtk...@mail.gmail.com
Signed-off-by: H. Peter Anvin h...@zytor.com
---
arch/x86/include/asm/uaccess.h | 28 +---
1 file changed, 17 insertions(+), 11 deletions(-)
diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index 8ec57c0..84ecf1d 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -40,22 +40,28 @@
/*
* Test whether a block of memory is a valid user space address.
* Returns 0 if the range is valid, nonzero otherwise.
- *
- * This is equivalent to the following test:
- * (u33)addr + (u33)size (u33)current-addr_limit.seg (u65 for x86_64)
- *
- * This needs 33-bit (65-bit for x86_64) arithmetic. We have a carry...
*/
+static inline int __chk_range_not_ok(unsigned long addr, unsigned long size,
unsigned long limit)
+{
+ /*
+* If we have used sizeof() for the size,
+* we know it won't overflow the limit (but
+* it might overflow the 'addr', so it's
+* important to subtract the size from the
+* limit, not add it to the address).
+*/
+ if (__builtin_constant_p(size))
+ return addr limit - size;
+
+ /* Arbitrary sizes? Be careful about overflow */
+ addr += size;
+ return (addr size) || (addr limit);
+}
#define __range_not_ok(addr, size, limit) \
({ \
- unsigned long flag, roksum; \
__chk_user_ptr(addr); \
- asm(add %3,%1 ; sbb %0,%0 ; cmp %1,%4 ; sbb $0,%0 \
- : =r (flag), =r (roksum) \
- : 1 (addr), g ((long)(size)), \
- rm (limit));\
- flag; \
+ __chk_range_not_ok((unsigned long __force)(addr), size, limit); \
})
/**
--
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