Subject: [PATCH v2 11/18] arm64: simplify access_ok()

From: Arnd Bergmann <[email-masked]>
Date: Wed, 16 Feb 2022 14:13:25 +0100

From: Arnd Bergmann <[email-masked]>

arm64 has an inline asm implementation of access_ok() that is derived from
the 32-bit arm version and optimized for the case that both the limit and
the size are variable. With set_fs() gone, the limit is always constant,
and the size usually is as well, so just using the default implementation
reduces the check into a comparison against a constant that can be
scheduled by the compiler.

On a defconfig build, this saves over 28KB of .text.

Acked-by: Robin Murphy <[email-masked]>
Acked-by: Mark Rutland <[email-masked]>
Signed-off-by: Arnd Bergmann <[email-masked]>
---
 arch/arm64/include/asm/uaccess.h | 34 ++++++++++----------------------
 1 file changed, 10 insertions(+), 24 deletions(-)

diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h
index 2e20879fe3cf..199c553b740a 100644
--- a/arch/arm64/include/asm/uaccess.h
+++ b/arch/arm64/include/asm/uaccess.h
@@ -26,6 +26,14 @@
 #include <asm/memory.h>
 #include <asm/extable.h>
 
+static inline int __access_ok(const void __user *ptr, unsigned long size)
+{
+	unsigned long limit = TASK_SIZE_MAX;
+	unsigned long addr = (unsigned long)ptr;
+
+	return (size <= limit) && (addr <= (limit - size));
+}
+
 /*
  * Test whether a block of memory is a valid user space address.
  * Returns 1 if the range is valid, 0 otherwise.
@@ -33,10 +41,8 @@
  * This is equivalent to the following test:
  * (u65)addr + (u65)size <= (u65)TASK_SIZE_MAX
  */
-static inline unsigned long __range_ok(const void __user *addr, unsigned long size)
+static inline int access_ok(const void __user *addr, unsigned long size)
 {
-	unsigned long ret, limit = TASK_SIZE_MAX - 1;
-
 	/*
 	 * Asynchronous I/O running in a kernel thread does not have the
 	 * TIF_TAGGED_ADDR flag of the process owning the mm, so always untag
@@ -46,29 +52,9 @@ static inline unsigned long __range_ok(const void __user *addr, unsigned long si
 	    (current->flags & PF_KTHREAD || test_thread_flag(TIF_TAGGED_ADDR)))
 		addr = untagged_addr(addr);
 
-	__chk_user_ptr(addr);
-	asm volatile(
-	// A + B <= C + 1 for all A,B,C, in four easy steps:
-	// 1: X = A + B; X' = X % 2^64
-	"	adds	%0, %3, %2\n"
-	// 2: Set C = 0 if X > 2^64, to guarantee X' > C in step 4
-	"	csel	%1, xzr, %1, hi\n"
-	// 3: Set X' = ~0 if X >= 2^64. For X == 2^64, this decrements X'
-	//    to compensate for the carry flag being set in step 4. For
-	//    X > 2^64, X' merely has to remain nonzero, which it does.
-	"	csinv	%0, %0, xzr, cc\n"
-	// 4: For X < 2^64, this gives us X' - C - 1 <= 0, where the -1
-	//    comes from the carry in being clear. Otherwise, we are
-	//    testing X' - C == 0, subject to the previous adjustments.
-	"	sbcs	xzr, %0, %1\n"
-	"	cset	%0, ls\n"
-	: "=&r" (ret), "+r" (limit) : "Ir" (size), "0" (addr) : "cc");
-
-	return ret;
+	return likely(__access_ok(addr, size));
 }
 
-#define access_ok(addr, size)	__range_ok(addr, size)
-
 /*
  * User access enabling/disabling.
  */
-- 
2.29.2