From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from lindbergh.monkeyblade.net (lindbergh.monkeyblade.net [23.128.96.19]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id E893E7E for ; Sat, 26 Aug 2023 04:24:33 +0000 (UTC) Received: from out-251.mta1.migadu.com (out-251.mta1.migadu.com [IPv6:2001:41d0:203:375::fb]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 82B5C269E for ; Fri, 25 Aug 2023 21:24:31 -0700 (PDT) Message-ID: <0aa436da-0b5a-166a-b45d-2ad11a985508@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1693023869; h=from:from:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=n/4vT8YbvwHuECViZGWyxb60rboGdv+dWEC3fQ+1Wrs=; b=LAB3dNYFxtL+CnXCIQsaFq/pZhmo80clBlKU78+YYnjT+vF4skZ1zpE+SQC+ZObdFEL5KI 2xMfn0Nb3/ZnwchgagpnGPY4crf7cQULXCZ6iOGW/RVhxsjuDJ5rn08XK5rFTUnQdWeHzL ZqtiZcDDCc0jNOsz5U+MMRYHE2caqwY= Date: Fri, 25 Aug 2023 21:24:23 -0700 Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Reply-To: yonghong.song@linux.dev Subject: Re: [PATCH bpf-next v2 1/1] docs/bpf: Add description for CO-RE relocations To: Eduard Zingerman , bpf@vger.kernel.org, ast@kernel.org Cc: andrii@kernel.org, daniel@iogearbox.net, martin.lau@linux.dev, kernel-team@fb.com References: <20230825224527.2465062-1-eddyz87@gmail.com> <20230825224527.2465062-2-eddyz87@gmail.com> Content-Language: en-US X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Yonghong Song In-Reply-To: <20230825224527.2465062-2-eddyz87@gmail.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT X-Spam-Status: No, score=-2.1 required=5.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,RCVD_IN_DNSWL_BLOCKED, SPF_HELO_NONE,SPF_PASS autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net On 8/25/23 3:45 PM, Eduard Zingerman wrote: > Add a section on CO-RE relocations to llvm_relo.rst. > Describe relevant .BTF.ext structure, `enum bpf_core_relo_kind` > and `struct bpf_core_relo` in some detail. > Description is based on doc-strings from: > - include/uapi/linux/bpf.h:struct bpf_core_relo > - tools/lib/bpf/relo_core.c:__bpf_core_types_match() > > Signed-off-by: Eduard Zingerman LGTM with a couple minor nits below. Acked-by: Yonghong Song > --- > Documentation/bpf/btf.rst | 31 +++- > Documentation/bpf/llvm_reloc.rst | 304 +++++++++++++++++++++++++++++++ > 2 files changed, 329 insertions(+), 6 deletions(-) > [...] > +.. code-block:: c > + > + enum bpf_core_relo_kind { > + BPF_CORE_FIELD_BYTE_OFFSET = 0, /* field byte offset */ > + BPF_CORE_FIELD_BYTE_SIZE = 1, /* field size in bytes */ > + BPF_CORE_FIELD_EXISTS = 2, /* field existence in target kernel */ > + BPF_CORE_FIELD_SIGNED = 3, /* field signedness (0 - unsigned, 1 - signed) */ > + BPF_CORE_FIELD_LSHIFT_U64 = 4, /* bitfield-specific left bitshift */ > + BPF_CORE_FIELD_RSHIFT_U64 = 5, /* bitfield-specific right bitshift */ > + BPF_CORE_TYPE_ID_LOCAL = 6, /* type ID in local BPF object */ > + BPF_CORE_TYPE_ID_TARGET = 7, /* type ID in target kernel */ > + BPF_CORE_TYPE_EXISTS = 8, /* type existence in target kernel */ > + BPF_CORE_TYPE_SIZE = 9, /* type size in bytes */ > + BPF_CORE_ENUMVAL_EXISTS = 10, /* enum value existence in target kernel */ > + BPF_CORE_ENUMVAL_VALUE = 11, /* enum value integer value */ > + BPF_CORE_TYPE_MATCHES = 12, /* type match in target kernel */ > + }; > + > +Notes: > + > +* ``BPF_CORE_FIELD_LSHIFT_U64`` and ``BPF_CORE_FIELD_RSHIFT_U64`` are > + supposed to be used to read bitfield values using the following > + algorithm: > + > + .. code-block:: c > + > + // To read bitfield ``f`` from ``struct s`` > + is_signed = relo(s->f, BPF_CORE_FIELD_SIGNED) > + off = relo(s->f, BPF_CORE_FIELD_BYTE_OFFSET) > + sz = relo(s->f, BPF_CORE_FIELD_BYTE_SIZE) > + l = relo(s->f, BPF_CORE_FIELD_LSHIFT_U64) > + r = relo(s->f, BPF_CORE_FIELD_RSHIFT_U64) > + // define ``v`` as signed or unsigned integer of size ``sz`` > + v = *((void *)s) + off) parenthesis not matching in the above. How about below to a little bit more precise? v = *({s|u} *)((void *)s + off) > + v <<= l > + v >>= r > + [...] > + > +CO-RE Relocation Examples > +========================= > + > +For the following C code: > + > +.. code-block:: c > + > + struct foo { > + int a; > + int b; > + unsigned c:15; > + } __attribute__((preserve_access_index)); > + > + enum bar { U, V }; > + > +With the following BTF definitions: > + > +.. code-block:: > + > + ... > + [2] STRUCT 'foo' size=8 vlen=2 > + 'a' type_id=3 bits_offset=0 > + 'b' type_id=3 bits_offset=32 > + 'c' type_id=4 bits_offset=64 bitfield_size=15 Misalignment in the above. > + [3] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED > + [4] INT 'unsigned int' size=4 bits_offset=0 nr_bits=32 encoding=(none) > + ... > + [16] ENUM 'bar' encoding=UNSIGNED size=4 vlen=2 > + 'U' val=0 > + 'V' val=1 > + > +Field offset relocations are generated automatically when > +``__attribute__((preserve_access_index))`` is used, for example: > + > +.. code-block:: c > + > + void alpha(struct foo *s, volatile unsigned long *g) { > + *g = s->a; > + s->a = 1; > + } > + > + 00 : > + 0: r3 = *(s32 *)(r1 + 0x0) > + 00: CO-RE [2] struct foo::a (0:0) > + 1: *(u64 *)(r2 + 0x0) = r3 > + 2: *(u32 *)(r1 + 0x0) = 0x1 > + 10: CO-RE [2] struct foo::a (0:0) > + 3: exit > + > + > +All relocation kinds could be requested via built-in functions. > +E.g. field-based relocations: > + > +.. code-block:: c > + > + void bravo(struct foo *s, volatile unsigned long *g) { > + *g = __builtin_preserve_field_info(s->b, 0 /* field byte offset */); > + *g = __builtin_preserve_field_info(s->b, 1 /* field byte size */); > + *g = __builtin_preserve_field_info(s->b, 2 /* field existence */); > + *g = __builtin_preserve_field_info(s->b, 3 /* field signedness */); > + *g = __builtin_preserve_field_info(s->c, 4 /* bitfield left shift */); > + *g = __builtin_preserve_field_info(s->c, 5 /* bitfield right shift */); > + } > + > + 20 : > + 4: r1 = 0x4 > + 20: CO-RE [2] struct foo::b (0:1) > + 5: *(u64 *)(r2 + 0x0) = r1 > + 6: r1 = 0x4 > + 30: CO-RE [2] struct foo::b (0:1) > + 7: *(u64 *)(r2 + 0x0) = r1 > + 8: r1 = 0x1 > + 40: CO-RE [2] struct foo::b (0:1) > + 9: *(u64 *)(r2 + 0x0) = r1 > + 10: r1 = 0x1 > + 50: CO-RE [2] struct foo::b (0:1) > + 11: *(u64 *)(r2 + 0x0) = r1 > + 12: r1 = 0x31 > + 60: CO-RE [2] struct foo::c (0:2) > + 13: *(u64 *)(r2 + 0x0) = r1 > + 14: r1 = 0x31 > + 70: CO-RE [2] struct foo::c (0:2) > + 15: *(u64 *)(r2 + 0x0) = r1 > + 16: exit > + > + [...]