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[24.56.227.58]) by smtp.gmail.com with ESMTPSA id 41be03b00d2f7-acebe856890sm2320390a12.42.2025.01.30.22.00.43 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 30 Jan 2025 22:00:43 -0800 (PST) Date: Thu, 30 Jan 2025 22:00:41 -0800 From: Charlie Jenkins To: Cyril Bur Cc: Jessica Clarke , palmer@dabbelt.com, aou@eecs.berkeley.edu, paul.walmsley@sifive.com, linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org, Jisheng Zhang Subject: Re: [PATCH v2 2/4] riscv: uaccess: use input constraints for ptr of __put_user Message-ID: References: <20241118230112.2872978-1-cyrilbur@tenstorrent.com> <20241118230112.2872978-3-cyrilbur@tenstorrent.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Fri, Jan 31, 2025 at 12:31:38PM +1100, Cyril Bur wrote: > > > On 18/1/2025 9:34 am, Jessica Clarke wrote: > > On 18 Nov 2024, at 23:01, Cyril Bur wrote: > > > > > > From: Jisheng Zhang > > > > > > I believe the output constraints "=m" is not necessary, because > > > the instruction itself is "write", we don't need the compiler > > > to "write" for us. So tell compiler we read from memory instead > > > of writing. > > > > That’s not what =m does. =m tells the compiler “this assembly will be > > writing to this memory location, and needs the address of the location > > for that operand”. If you move it from an output to an input then you > > get the same assembly generated, but the compiler believes you are > > *reading* from that memory, not *writing* to it, and so does not > > believe the memory may be clobbered. > > > > Now, it may well be that, by virtue of this being userspace memory that > > is only ever accessed via assembly, and any inline assembly being > > marked volatile, this in effect doesn’t matter. But it is still > > technically wrong to model it that way, and you cannot use the > > justification you are using, because it is false, and demonstrates a > > lack of understanding for how inline assembly works. It has to be > > correctly justified. > > > > (I do note that neither x86 nor arm64 seems to model this as an output) > > > > Jess > > Good points, I agree. I am happy to respin this patch > move the =m to output constraint. > > @Charlie Please let me know if you would like me to > keep your tags on the tweaked version. Can you remove the tags? Copy me on the next version and I will review it again. - Charlie > > Thanks, > > Cyril > > > > > Signed-off-by: Jisheng Zhang > > > Signed-off-by: Cyril Bur > > > --- > > > arch/riscv/include/asm/uaccess.h | 18 +++++++++--------- > > > 1 file changed, 9 insertions(+), 9 deletions(-) > > > > > > diff --git a/arch/riscv/include/asm/uaccess.h b/arch/riscv/include/asm/uaccess.h > > > index 09d4ca37522c..84b084e388a7 100644 > > > --- a/arch/riscv/include/asm/uaccess.h > > > +++ b/arch/riscv/include/asm/uaccess.h > > > @@ -186,11 +186,11 @@ do { \ > > > __typeof__(*(ptr)) __x = x; \ > > > __asm__ __volatile__ ( \ > > > "1:\n" \ > > > - " " insn " %z2, %1\n" \ > > > + " " insn " %z1, %2\n" \ > > > "2:\n" \ > > > _ASM_EXTABLE_UACCESS_ERR(1b, 2b, %0) \ > > > - : "+r" (err), "=m" (*(ptr)) \ > > > - : "rJ" (__x)); \ > > > + : "+r" (err) \ > > > + : "rJ" (__x), "m"(*(ptr))); \ > > > } while (0) > > > > > > #ifdef CONFIG_64BIT > > > @@ -203,16 +203,16 @@ do { \ > > > u64 __x = (__typeof__((x)-(x)))(x); \ > > > __asm__ __volatile__ ( \ > > > "1:\n" \ > > > - " sw %z3, %1\n" \ > > > + " sw %z1, %3\n" \ > > > "2:\n" \ > > > - " sw %z4, %2\n" \ > > > + " sw %z2, %4\n" \ > > > "3:\n" \ > > > _ASM_EXTABLE_UACCESS_ERR(1b, 3b, %0) \ > > > _ASM_EXTABLE_UACCESS_ERR(2b, 3b, %0) \ > > > - : "+r" (err), \ > > > - "=m" (__ptr[__LSW]), \ > > > - "=m" (__ptr[__MSW]) \ > > > - : "rJ" (__x), "rJ" (__x >> 32)); \ > > > + : "+r" (err) \ > > > + : "rJ" (__x), "rJ" (__x >> 32), \ > > > + "m" (__ptr[__LSW]), \ > > > + "m" (__ptr[__MSW])); \ > > > } while (0) > > > #endif /* CONFIG_64BIT */ > > > > > > -- > > > 2.34.1 > > > > > > > > > _______________________________________________ > > > linux-riscv mailing list > > > linux-riscv@lists.infradead.org > > > http://lists.infradead.org/mailman/listinfo/linux-riscv > > >