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Message-ID: <CAMj1kXHixUFjV=4m3tzfGz7AiRWc-VczymbKuZq7dyZZNuLKxQ@mail.gmail.com>
Date: Tue, 15 Feb 2022 09:17:48 +0100
From: Ard Biesheuvel <ardb@...nel.org>
To: Arnd Bergmann <arnd@...nel.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>,
Christoph Hellwig <hch@....de>,
linux-arch <linux-arch@...r.kernel.org>,
Linux Memory Management List <linux-mm@...ck.org>,
linux-api@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
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Robin Murphy <robin.murphy@....com>
Subject: Re: [PATCH 08/14] arm64: simplify access_ok()
On Mon, 14 Feb 2022 at 17:37, Arnd Bergmann <arnd@...nel.org> wrote:
>
> From: Arnd Bergmann <arnd@...db.de>
>
> 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.
>
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
> arch/arm64/include/asm/uaccess.h | 28 +++++-----------------------
> 1 file changed, 5 insertions(+), 23 deletions(-)
>
> diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h
> index 357f7bd9c981..e8dce0cc5eaa 100644
> --- a/arch/arm64/include/asm/uaccess.h
> +++ b/arch/arm64/include/asm/uaccess.h
> @@ -26,6 +26,8 @@
> #include <asm/memory.h>
> #include <asm/extable.h>
>
> +static inline int __access_ok(const void __user *ptr, unsigned long size);
> +
> /*
> * Test whether a block of memory is a valid user space address.
> * Returns 1 if the range is valid, 0 otherwise.
> @@ -33,10 +35,8 @@
> * This is equivalent to the following test:
> * (u65)addr + (u65)size <= (u65)TASK_SIZE_MAX
> */
> -static inline unsigned long __access_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
> @@ -46,27 +46,9 @@ static inline unsigned long __access_ok(const void __user *addr, unsigned long s
> (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 __access_ok
> +#define access_ok access_ok
>
> #include <asm-generic/access_ok.h>
>
> --
> 2.29.2
>
With set_fs() out of the picture, wouldn't it be sufficient to check
that bit #55 is clear? (the bit that selects between TTBR0 and TTBR1)
That would also remove the need to strip the tag from the address.
Something like
asm goto("tbnz %0, #55, %2 \n"
"tbnz %1, #55, %2 \n"
:: "r"(addr), "r"(addr + size - 1) :: notok);
return 1;
notok:
return 0;
with an additional sanity check on the size which the compiler could
eliminate for compile-time constant values.
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