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Date: Mon, 14 Dec 2020 11:51:16 +0800
From: Gary Lin <glin@...e.com>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Eric Dumazet <eric.dumazet@...il.com>,
andreas.taschner@...e.com
Subject: Re: [PATCH] bpf,x64: pad NOPs to make images converge more easily
On Fri, Dec 11, 2020 at 12:58:17PM -0800, Andrii Nakryiko wrote:
> On Fri, Dec 11, 2020 at 8:51 AM Gary Lin <glin@...e.com> wrote:
> >
> > The x64 bpf jit expects bpf images converge within the given passes, but
> > it could fail to do so with some corner cases. For example:
> >
> > l0: ldh [4]
> > l1: jeq #0x537d, l2, l40
> > l2: ld [0]
> > l3: jeq #0xfa163e0d, l4, l40
> > l4: ldh [12]
> > l5: ldx #0xe
> > l6: jeq #0x86dd, l41, l7
> > l8: ld [x+16]
> > l9: ja 41
> >
> > [... repeated ja 41 ]
> >
> > l40: ja 41
> > l41: ret #0
> > l42: ld #len
> > l43: ret a
> >
> > This bpf program contains 32 "ja 41" instructions which are effectively
> > NOPs and designed to be replaced with valid code dynamically. Ideally,
> > bpf jit should optimize those "ja 41" instructions out when translating
> > the bpf instructions into x86_64 machine code. However, do_jit() can
> > only remove one "ja 41" for offset==0 on each pass, so it requires at
> > least 32 runs to eliminate those JMPs and exceeds the current limit of
> > passes (20). In the end, the program got rejected when BPF_JIT_ALWAYS_ON
> > is set even though it's legit as a classic socket filter.
> >
> > To make the image more likely converge within 20 passes, this commit
> > pads some instructions with NOPs in the last 5 passes:
> >
> > 1. conditional jumps
> > A possible size variance comes from the adoption of imm8 JMP. If the
> > offset is imm8, we calculate the size difference of this BPF instruction
> > between the previous pass and the current pass and fill the gap with NOPs.
> > To avoid the recalculation of jump offset, those NOPs are inserted before
> > the JMP code, so we have to subtract the 2 bytes of imm8 JMP when
> > calculating the NOP number.
> >
> > 2. BPF_JA
> > There are two conditions for BPF_JA.
> > a.) nop jumps
> > If this instruction is not optimized out in the previous pass,
> > instead of removing it, we insert the equivalent size of NOPs.
> > b.) label jumps
> > Similar to condition jumps, we prepend NOPs right before the JMP
> > code.
> >
> > To make the code concise, emit_nops() is modified to use the signed len and
> > return the number of inserted NOPs.
> >
> > To support bpf-to-bpf, a new flag, padded, is introduced to 'struct bpf_prog'
> > so that bpf_int_jit_compile() could know if the program is padded or not.
> >
> > Signed-off-by: Gary Lin <glin@...e.com>
> > ---
> > arch/x86/net/bpf_jit_comp.c | 68 ++++++++++++++++++++++++-------------
> > include/linux/filter.h | 1 +
> > 2 files changed, 45 insertions(+), 24 deletions(-)
> >
> > diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> > index 796506dcfc42..30b81c8539b3 100644
> > --- a/arch/x86/net/bpf_jit_comp.c
> > +++ b/arch/x86/net/bpf_jit_comp.c
> > @@ -789,8 +789,31 @@ static void detect_reg_usage(struct bpf_insn *insn, int insn_cnt,
> > }
> > }
> >
> > +static int emit_nops(u8 **pprog, int len)
> > +{
> > + u8 *prog = *pprog;
> > + int i, noplen, cnt = 0;
> > +
> > + while (len > 0) {
> > + noplen = len;
> > +
> > + if (noplen > ASM_NOP_MAX)
> > + noplen = ASM_NOP_MAX;
> > +
> > + for (i = 0; i < noplen; i++)
> > + EMIT1(ideal_nops[noplen][i]);
> > + len -= noplen;
> > + }
> > +
> > + *pprog = prog;
> > +
> > + return cnt;
>
> Isn't cnt always zero? I guess it was supposed to be `cnt = len` at
> the beginning?
>
[Skip this one since Daniel answered in another mail.]
> But then it begs the question how this patch was actually tested given
> emit_nops() is returning wrong answers? Changes like this should
> definitely come with tests.
>
Before submitting this patch, I lowered PADDING_PASSES to 2 and ran
tools/testing/selftests/bpf/test_progs. We surely need some official
test cases for this patch. Will do it in v2.
> > +}
> > +
> > +#define INSN_SZ_DIFF (((addrs[i] - addrs[i - 1]) - (prog - temp)))
> > +
> > static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image,
> > - int oldproglen, struct jit_context *ctx)
> > + int oldproglen, struct jit_context *ctx, bool jmp_padding)
> > {
> > bool tail_call_reachable = bpf_prog->aux->tail_call_reachable;
> > struct bpf_insn *insn = bpf_prog->insnsi;
> > @@ -1409,6 +1432,8 @@ xadd: if (is_imm8(insn->off))
> > }
> > jmp_offset = addrs[i + insn->off] - addrs[i];
> > if (is_imm8(jmp_offset)) {
> > + if (jmp_padding)
> > + cnt += emit_nops(&prog, INSN_SZ_DIFF - 2);
> > EMIT2(jmp_cond, jmp_offset);
> > } else if (is_simm32(jmp_offset)) {
> > EMIT2_off32(0x0F, jmp_cond + 0x10, jmp_offset);
> > @@ -1431,11 +1456,19 @@ xadd: if (is_imm8(insn->off))
> > else
> > jmp_offset = addrs[i + insn->off] - addrs[i];
> >
> > - if (!jmp_offset)
> > - /* Optimize out nop jumps */
> > + if (!jmp_offset) {
> > + /*
> > + * If jmp_padding is enabled, the extra nops will
> > + * be inserted. Otherwise, optimize out nop jumps.
> > + */
> > + if (jmp_padding)
> > + cnt += emit_nops(&prog, INSN_SZ_DIFF);
> > break;
> > + }
> > emit_jmp:
> > if (is_imm8(jmp_offset)) {
> > + if (jmp_padding)
> > + cnt += emit_nops(&prog, INSN_SZ_DIFF - 2);
> > EMIT2(0xEB, jmp_offset);
> > } else if (is_simm32(jmp_offset)) {
> > EMIT1_off32(0xE9, jmp_offset);
> > @@ -1578,26 +1611,6 @@ static int invoke_bpf_prog(const struct btf_func_model *m, u8 **pprog,
> > return 0;
> > }
> >
> > -static void emit_nops(u8 **pprog, unsigned int len)
> > -{
> > - unsigned int i, noplen;
> > - u8 *prog = *pprog;
> > - int cnt = 0;
> > -
> > - while (len > 0) {
> > - noplen = len;
> > -
> > - if (noplen > ASM_NOP_MAX)
> > - noplen = ASM_NOP_MAX;
> > -
> > - for (i = 0; i < noplen; i++)
> > - EMIT1(ideal_nops[noplen][i]);
> > - len -= noplen;
> > - }
> > -
> > - *pprog = prog;
> > -}
> > -
> > static void emit_align(u8 **pprog, u32 align)
> > {
> > u8 *target, *prog = *pprog;
> > @@ -1972,6 +1985,9 @@ struct x64_jit_data {
> > struct jit_context ctx;
> > };
> >
> > +#define MAX_PASSES 20
> > +#define PADDING_PASSES (MAX_PASSES - 5)
> > +
> > struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
> > {
> > struct bpf_binary_header *header = NULL;
> > @@ -1981,6 +1997,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
> > struct jit_context ctx = {};
> > bool tmp_blinded = false;
> > bool extra_pass = false;
> > + bool padding = prog->padded;
>
> can this ever be true on assignment? I.e., can the program be jitted twice?
>
Yes, bpf-to-bpf runs bpf_int_jit_compile twice and it expects to run
only one pass in the second time.
> > u8 *image = NULL;
> > int *addrs;
> > int pass;
> > @@ -2043,7 +2060,9 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
> > * pass to emit the final image.
> > */
> > for (pass = 0; pass < 20 || image; pass++) {
> > - proglen = do_jit(prog, addrs, image, oldproglen, &ctx);
> > + if (!padding && pass >= PADDING_PASSES)
> > + padding = true;
>
> Just, unconditionally:
>
> padding = pass >= PADDING_PASSES;
>
But this would turn 'padding' from 'true' to 'false' when invoking
bpf_int_jit_compile() the second time for bpf-to-bpf, so we still need
to do it conditionally.
Gary Lin
> > + proglen = do_jit(prog, addrs, image, oldproglen, &ctx, padding);
> > if (proglen <= 0) {
> > out_image:
> > image = NULL;
> > @@ -2101,6 +2120,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
> > prog->bpf_func = (void *)image;
> > prog->jited = 1;
> > prog->jited_len = proglen;
> > + prog->padded = padding;
> > } else {
> > prog = orig_prog;
> > }
> > diff --git a/include/linux/filter.h b/include/linux/filter.h
> > index 1b62397bd124..cb7ce2b3737a 100644
> > --- a/include/linux/filter.h
> > +++ b/include/linux/filter.h
> > @@ -531,6 +531,7 @@ struct bpf_prog {
> > dst_needed:1, /* Do we need dst entry? */
> > blinded:1, /* Was blinded */
> > is_func:1, /* program is a bpf function */
> > + padded:1, /* jitted image was padded */
> > kprobe_override:1, /* Do we override a kprobe? */
> > has_callchain_buf:1, /* callchain buffer allocated? */
> > enforce_expected_attach_type:1, /* Enforce expected_attach_type checking at attach time */
> > --
> > 2.29.2
> >
>
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