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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>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Russell King <linux@...linux.org.uk>, Will Deacon <will@...nel.org>, Guo Ren <guoren@...nel.org>, Brian Cain <bcain@...eaurora.org>, Geert Uytterhoeven <geert@...ux-m68k.org>, monstr@...str.eu, Thomas Bogendoerfer <tsbogend@...ha.franken.de>, Nick Hu <nickhu@...estech.com>, Greentime Hu <green.hu@...il.com>, dinguyen@...nel.org, Stafford Horne <shorne@...il.com>, Helge Deller <deller@....de>, Michael Ellerman <mpe@...erman.id.au>, Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, Mark Rutland <mark.rutland@....com>, Heiko Carstens <hca@...ux.ibm.com>, Rich Felker <dalias@...c.org>, "David S. Miller" <davem@...emloft.net>, Richard Weinberger <richard@....at>, X86 ML <x86@...nel.org>, Max Filippov <jcmvbkbc@...il.com>, "Eric W. Biederman" <ebiederm@...ssion.com>, Andrew Morton <akpm@...ux-foundation.org>, alpha <linux-alpha@...r.kernel.org>, linux-snps-arc@...ts.infradead.org, Linux ARM <linux-arm-kernel@...ts.infradead.org>, linux-csky@...r.kernel.org, linux-hexagon@...r.kernel.org, linux-ia64@...r.kernel.org, linux-m68k <linux-m68k@...ts.linux-m68k.org>, "open list:MIPS" <linux-mips@...r.kernel.org>, openrisc@...ts.librecores.org, "open list:PARISC ARCHITECTURE" <linux-parisc@...r.kernel.org>, "open list:LINUX FOR POWERPC (32-BIT AND 64-BIT)" <linuxppc-dev@...ts.ozlabs.org>, linux-riscv <linux-riscv@...ts.infradead.org>, "open list:S390" <linux-s390@...r.kernel.org>, linux-sh@...r.kernel.org, "open list:SPARC + UltraSPARC (sparc/sparc64)" <sparclinux@...r.kernel.org>, linux-um <linux-um@...ts.infradead.org>, "open list:TENSILICA XTENSA PORT (xtensa)" <linux-xtensa@...ux-xtensa.org>, 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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