From: Anton Protopopov <a.s.protopopov@gmail.com>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: bpf@vger.kernel.org, daniel@iogearbox.net, andrii@kernel.org,
eddyz87@gmail.com, memxor@gmail.com
Subject: Re: [PATCH v2 bpf-next 06/13] resolve_btfids: Fold the constants of a select without a branch
Date: Fri, 13 Nov 2026 11:59:45 +0000 [thread overview]
Message-ID: <avb7sY_yAZuvTnxZ@mail.gmail.com> (raw)
In-Reply-To: <20261113050901.1399091-7-alexei.starovoitov@gmail.com>
On 26/11/13 05:08AM, Alexei Starovoitov wrote:
> From: Alexei Starovoitov <ast@kernel.org>
>
> return copy_to_user(to, from, n) ? -EFAULT : 0;
>
> GCC for x86-64 never loads -14 into a register here:
>
> neg %rax
> sbb %eax,%eax 0 or -1
> and $0xfffffff2,%eax 0 or -14
> ret
>
> So the function gets no tag. Other forms of the same select:
>
> one of two errors sbb; and; sub
> the condition is negated sbb; not; and
>
> The table of x86-64 describes "sbb %eax,%eax" as a definition with two
> constants, 0 and -1. Describe "dst = src op imm" in text_insn.h as
> TEXT_ALU_IMM. text_flow_fold() runs before the analysis. It computes the
> instruction if src still holds the constants of an earlier instruction,
> and the code falls through from there. The result is again a definition
> with two constants.
>
> Five patterns of x86-64 use it: "and" and "sub" of a short immediate, in
> 32 and in 64 bits, and "not".
>
> functions tagged before after
> x86-64 GCC 12633 12817
> x86-64 clang 13541 13549
> arm64 clang 12435 12435
>
> Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Anton Protopopov <a.s.protopopov@gmail.com>
> [...]
next prev parent reply other threads:[~2026-11-13 11:48 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-11-13 5:08 [PATCH v2 bpf-next 00/13] bpf: fmod_ret for each function that returns errors Alexei Starovoitov
2026-11-13 5:08 ` [PATCH v2 bpf-next 01/13] selftests/bpf: Don't depend on the width of a BTF id in verifier_live_stack Alexei Starovoitov
2026-11-13 11:13 ` Anton Protopopov
2026-11-13 5:08 ` [PATCH v2 bpf-next 02/13] resolve_btfids: Read the code of an ELF file by address Alexei Starovoitov
2026-11-13 5:08 ` [PATCH v2 bpf-next 03/13] resolve_btfids: Describe machine instructions with LLVM's MC layer Alexei Starovoitov
2026-11-13 11:45 ` Anton Protopopov
2026-11-13 5:08 ` [PATCH v2 bpf-next 04/13] resolve_btfids: Find the definitions that reach the return value Alexei Starovoitov
2026-11-13 5:08 ` [PATCH v2 bpf-next 05/13] resolve_btfids: Tag the functions that return errors Alexei Starovoitov
2026-11-13 5:08 ` [PATCH v2 bpf-next 06/13] resolve_btfids: Fold the constants of a select without a branch Alexei Starovoitov
2026-11-13 11:59 ` Anton Protopopov [this message]
2026-11-13 5:08 ` [PATCH v2 bpf-next 07/13] resolve_btfids: Say which registers an instruction reads Alexei Starovoitov
2026-11-13 12:03 ` Anton Protopopov
2026-11-13 5:08 ` [PATCH v2 bpf-next 08/13] resolve_btfids: Don't list an error that follows work on an argument Alexei Starovoitov
2026-11-13 5:52 ` bot+bpf-ci
2026-11-13 5:08 ` [PATCH v2 bpf-next 09/13] bpf: Allow fmod_ret on tagged functions and check the return value Alexei Starovoitov
2026-11-13 5:52 ` bot+bpf-ci
2026-11-13 5:08 ` [PATCH v2 bpf-next 10/13] selftests/bpf: Test fmod_ret on tagged functions Alexei Starovoitov
2026-11-13 5:08 ` [PATCH v2 bpf-next 11/13] bpf: Mark the arguments of fmod_ret progs as PTR_UNTRUSTED Alexei Starovoitov
2026-11-13 11:09 ` Anton Protopopov
2026-11-13 5:09 ` [PATCH v2 bpf-next 12/13] bpf: Treat the type of an untrusted pointer as a hint Alexei Starovoitov
2026-11-13 5:52 ` bot+bpf-ci
2026-11-13 5:09 ` [PATCH v2 bpf-next 13/13] selftests/bpf: Test untrusted pointers in fmod_ret progs Alexei Starovoitov
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