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Fri, 13 Nov 2026 03:52:21 -0800 (PST) Received: from mail.gmail.com ([2a04:ee41:4:b2de:1ac0:4dff:fe0f:3782]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-49c605e4a2asm5012590f8f.39.2026.11.13.03.52.20 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 13 Nov 2026 03:52:21 -0800 (PST) Date: Fri, 13 Nov 2026 12:03:58 +0000 From: Anton Protopopov To: Alexei Starovoitov Cc: bpf@vger.kernel.org, daniel@iogearbox.net, andrii@kernel.org, eddyz87@gmail.com, memxor@gmail.com Subject: Re: [PATCH v2 bpf-next 07/13] resolve_btfids: Say which registers an instruction reads Message-ID: References: <20261113050901.1399091-1-alexei.starovoitov@gmail.com> <20261113050901.1399091-8-alexei.starovoitov@gmail.com> X-Mailing-List: bpf@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261113050901.1399091-8-alexei.starovoitov@gmail.com> On 26/11/13 05:08AM, Alexei Starovoitov wrote: > From: Alexei Starovoitov > > Ebusy: > spin_unlock(&files->file_lock); > return -EBUSY; > > mov %rdi,%r13 files, the first argument > ... > lea 0xc0(%r13),%rdi &files->file_lock > call _raw_spin_unlock > mov $0xfffffff0,%eax > > do_dup2() unlocks an object of its caller before it returns the error. The > next patch looks for that in the machine code: is the pointer in %rdi > computed from an argument of do_dup2()? The decoder says which registers > an instruction writes. It does not say from what. > > text_insn.uses the registers that the instruction reads: the > operands behind the defs of MCInstrDesc, and its > implicit uses > text_insn.load it reads memory: MCInstrDesc::mayLoad() > > lea 0xc0(%r13),%rdi uses r13: a pointer into *files > mov 0xc0(%r13),%rdi uses r13, load: another object > > An opcode with hasUnmodeledSideEffects() reads all registers, as it writes > all registers. > > For LLVM "xor %edi,%edi" reads %edi. Compilers pass NULL in this way. The > table of x86-64 gets a pattern for it: dst = 0. > > A test instruction says what is read only if that is its point: four for > x86-64, three for arm64. > > No tag changes. > > Signed-off-by: Alexei Starovoitov Acked-by: Anton Protopopov > [...]