From: John Fastabend <john.fastabend@gmail.com>
To: Brendan Jackman <jackmanb@google.com>, bpf@vger.kernel.org
Cc: Alexei Starovoitov <ast@kernel.org>, Yonghong Song <yhs@fb.com>,
Daniel Borkmann <daniel@iogearbox.net>,
KP Singh <kpsingh@chromium.org>,
Florent Revest <revest@chromium.org>,
linux-kernel@vger.kernel.org, Jann Horn <jannh@google.com>,
Brendan Jackman <jackmanb@google.com>
Subject: RE: [PATCH bpf-next v4 07/11] bpf: Add instructions for atomic_[cmp]xchg
Date: Mon, 07 Dec 2020 22:42:02 -0800 [thread overview]
Message-ID: <5fcf203ab087a_d22720855@john-XPS-13-9370.notmuch> (raw)
In-Reply-To: <20201207160734.2345502-8-jackmanb@google.com>
Brendan Jackman wrote:
> This adds two atomic opcodes, both of which include the BPF_FETCH
> flag. XCHG without the BPF_FETCH flag would naturally encode
> atomic_set. This is not supported because it would be of limited
> value to userspace (it doesn't imply any barriers). CMPXCHG without
> BPF_FETCH woulud be an atomic compare-and-write. We don't have such
> an operation in the kernel so it isn't provided to BPF either.
>
> There are two significant design decisions made for the CMPXCHG
> instruction:
>
> - To solve the issue that this operation fundamentally has 3
> operands, but we only have two register fields. Therefore the
> operand we compare against (the kernel's API calls it 'old') is
> hard-coded to be R0. x86 has similar design (and A64 doesn't
> have this problem).
>
> A potential alternative might be to encode the other operand's
> register number in the immediate field.
>
> - The kernel's atomic_cmpxchg returns the old value, while the C11
> userspace APIs return a boolean indicating the comparison
> result. Which should BPF do? A64 returns the old value. x86 returns
> the old value in the hard-coded register (and also sets a
> flag). That means return-old-value is easier to JIT.
>
> Signed-off-by: Brendan Jackman <jackmanb@google.com>
> ---
Sorry if this is a dup, client crashed while I sent the previous version
and don't see it on the list.
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -3608,11 +3608,14 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regn
>
> static int check_atomic(struct bpf_verifier_env *env, int insn_idx, struct bpf_insn *insn)
> {
> + int load_reg;
> int err;
>
> switch (insn->imm) {
> case BPF_ADD:
> case BPF_ADD | BPF_FETCH:
> + case BPF_XCHG:
> + case BPF_CMPXCHG:
> break;
> default:
> verbose(env, "BPF_ATOMIC uses invalid atomic opcode %02x\n", insn->imm);
> @@ -3634,6 +3637,13 @@ static int check_atomic(struct bpf_verifier_env *env, int insn_idx, struct bpf_i
> if (err)
> return err;
>
> + if (insn->imm == BPF_CMPXCHG) {
> + /* Check comparison of R0 with memory location */
> + err = check_reg_arg(env, BPF_REG_0, SRC_OP);
> + if (err)
> + return err;
> + }
> +
Need to think a bit more on this, but do we need to update is_reg64() here
as well?
> if (is_pointer_value(env, insn->src_reg)) {
> verbose(env, "R%d leaks addr into mem\n", insn->src_reg);
> return -EACCES;
> @@ -3664,8 +3674,13 @@ static int check_atomic(struct bpf_verifier_env *env, int insn_idx, struct bpf_i
> if (!(insn->imm & BPF_FETCH))
> return 0;
>
> - /* check and record load of old value into src reg */
> - err = check_reg_arg(env, insn->src_reg, DST_OP);
> + if (insn->imm == BPF_CMPXCHG)
> + load_reg = BPF_REG_0;
> + else
> + load_reg = insn->src_reg;
> +
> + /* check and record load of old value */
> + err = check_reg_arg(env, load_reg, DST_OP);
> if (err)
> return err;
>
Thanks,
John
next prev parent reply other threads:[~2020-12-08 6:43 UTC|newest]
Thread overview: 40+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-07 16:07 [PATCH bpf-next v4 00/11] Atomics for eBPF Brendan Jackman
2020-12-07 16:07 ` [PATCH bpf-next v4 01/11] bpf: x86: Factor out emission of ModR/M for *(reg + off) Brendan Jackman
2020-12-07 21:04 ` John Fastabend
2020-12-07 16:07 ` [PATCH bpf-next v4 02/11] bpf: x86: Factor out emission of REX byte Brendan Jackman
2020-12-07 21:07 ` John Fastabend
2020-12-07 16:07 ` [PATCH bpf-next v4 03/11] bpf: x86: Factor out a lookup table for some ALU opcodes Brendan Jackman
2020-12-07 21:08 ` John Fastabend
2020-12-07 16:07 ` [PATCH bpf-next v4 04/11] bpf: Rename BPF_XADD and prepare to encode other atomics in .imm Brendan Jackman
2020-12-07 21:56 ` John Fastabend
2020-12-08 9:26 ` Brendan Jackman
2020-12-09 5:40 ` John Fastabend
2020-12-07 16:07 ` [PATCH bpf-next v4 05/11] bpf: Move BPF_STX reserved field check into BPF_STX verifier code Brendan Jackman
2020-12-08 1:35 ` Yonghong Song
2020-12-08 5:13 ` John Fastabend
2020-12-07 16:07 ` [PATCH bpf-next v4 06/11] bpf: Add BPF_FETCH field / create atomic_fetch_add instruction Brendan Jackman
2020-12-08 1:41 ` Yonghong Song
2020-12-08 9:31 ` Brendan Jackman
2020-12-08 5:31 ` John Fastabend
2020-12-08 9:59 ` Brendan Jackman
2020-12-07 16:07 ` [PATCH bpf-next v4 07/11] bpf: Add instructions for atomic_[cmp]xchg Brendan Jackman
2020-12-08 1:44 ` Yonghong Song
2020-12-08 6:37 ` John Fastabend
2020-12-14 15:39 ` Brendan Jackman
2020-12-08 6:42 ` John Fastabend [this message]
2020-12-07 16:07 ` [PATCH bpf-next v4 08/11] bpf: Pull out a macro for interpreting atomic ALU operations Brendan Jackman
2020-12-07 16:07 ` [PATCH bpf-next v4 09/11] bpf: Add bitwise atomic instructions Brendan Jackman
2020-12-08 1:47 ` Yonghong Song
2020-12-10 0:22 ` kernel test robot
2020-12-07 16:07 ` [PATCH bpf-next v4 10/11] bpf: Add tests for new BPF atomic operations Brendan Jackman
2020-12-08 3:18 ` Yonghong Song
2020-12-08 12:41 ` Brendan Jackman
2020-12-08 16:38 ` Yonghong Song
2020-12-08 16:59 ` Brendan Jackman
2020-12-08 18:15 ` Yonghong Song
2020-12-15 11:12 ` Brendan Jackman
2020-12-16 7:18 ` Yonghong Song
2020-12-16 11:51 ` Brendan Jackman
2020-12-16 20:00 ` Yonghong Song
2020-12-07 16:07 ` [PATCH bpf-next v4 11/11] bpf: Document new atomic instructions Brendan Jackman
2020-12-08 3:25 ` Yonghong Song
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=5fcf203ab087a_d22720855@john-XPS-13-9370.notmuch \
--to=john.fastabend@gmail.com \
--cc=ast@kernel.org \
--cc=bpf@vger.kernel.org \
--cc=daniel@iogearbox.net \
--cc=jackmanb@google.com \
--cc=jannh@google.com \
--cc=kpsingh@chromium.org \
--cc=linux-kernel@vger.kernel.org \
--cc=revest@chromium.org \
--cc=yhs@fb.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).