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Thu, 15 Oct 2026 19:46:05 -0700 (PDT) Content-Type: text/plain; charset=UTF-8 Date: Fri, 16 Oct 2026 02:46:05 +0000 Message-Id: Cc: "Andrii Nakryiko" , "Daniel Borkmann" , "Eduard Zingerman" , Subject: Re: [PATCH bpf-next v3 10/11] bpf, x86: Give a kfunc's stack argument the 16-byte alignment it needs From: "Alexei Starovoitov" To: "Yonghong Song" , In-Reply-To: <20261013171035.2137865-1-yonghong.song@linux.dev> References: <20261013170910.2126673-1-yonghong.song@linux.dev> <20261013171035.2137865-1-yonghong.song@linux.dev> X-Mailer: mkdraft (claude review draft; edit before sending) Content-Transfer-Encoding: 8bit Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Tue, Oct 13, 2026 at 10:10 AM Yonghong Song wrote: > The JIT places a kfunc's stack arguments at the offsets from RSP that > the convention gives them, holes included, but RSP itself is only 8-byte > aligned in a BPF program, so a kfunc can get such an argument at an > address that is not 16-byte aligned. There are no kfuncs that take __int128 on the stack. The kernel is built with -mno-sse, so nothing but cmpxchg16b on &g cares about this alignment. Can we drop this patch? because... > + for (i = 0; i < n; i++) > + EMIT4(0xFF, 0x74, 0x24, disp); /* push qword ptr [rsp + disp] */ > + skip[-1] = prog - skip; u64 kfunc(u64 a, u64 b, u64 c, u64 d, u64 e, __int128 x, __int128 y, __int128 z, u64 w) takes 133 bytes total. BPF_MAX_INSN_SIZE is 128. do_jit() prints "bpf_jit: fatal insn size error" and the prog fails to load. It was 92 bytes before this patch. pw-bot: cr