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Subject: Re: [PATCH bpf-next,v2] riscv, bpf: Add BPF exception tables
To:     =?UTF-8?B?QmrDtnJuIFTDtnBlbA==?= <bjorn.topel@gmail.com>
References: <20211025035324.517263-1-tongtiangen@huawei.com>
 <CAJ+HfNgSVSs5BxVdDHuGNXnC9dxTuZVtx5RiPX=O8okTJZwsPw@mail.gmail.com>
CC:     Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmerdabbelt@google.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Luke Nelson <luke.r.nels@gmail.com>,
        Xi Wang <xi.wang@gmail.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        linux-riscv <linux-riscv@lists.infradead.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Netdev <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>
From:   tongtiangen <tongtiangen@huawei.com>
Message-ID: <4ab2e551-e371-2e94-dce2-432be3c09061@huawei.com>
Date:   Tue, 26 Oct 2021 15:51:51 +0800
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On 2021/10/26 2:27, Björn Töpel wrote:
> On Mon, 25 Oct 2021 at 05:38, Tong Tiangen <tongtiangen@huawei.com> wrote:
>>
>> When a tracing BPF program attempts to read memory without using the
>> bpf_probe_read() helper, the verifier marks the load instruction with
>> the BPF_PROBE_MEM flag. Since the riscv JIT does not currently recognize
>> this flag it falls back to the interpreter.
>>
>> Add support for BPF_PROBE_MEM, by appending an exception table to the
>> BPF program. If the load instruction causes a data abort, the fixup
>> infrastructure finds the exception table and fixes up the fault, by
>> clearing the destination register and jumping over the faulting
>> instruction.
>>
>> A more generic solution would add a "handler" field to the table entry,
>> like on x86 and s390.
>>
>> The same issue in ARM64 is fixed in:
>> commit 800834285361 ("bpf, arm64: Add BPF exception tables")
>>
>> Signed-off-by: Tong Tiangen <tongtiangen@huawei.com>
>> Tested-by: Pu Lehui <pulehui@huawei.com>
>> ---
>> v2:
>> Modify according to Björn's comments, mainly removes redundant head files
>> extable.h and some code style issues.
>>
>
> Thanks Tong! I haven't got around to take it for a spin yet.
>
> However, some more minor nits, and some other comments.

Thanks Björn, Your comments are very useful.

>
>>  arch/riscv/mm/extable.c         |  27 ++++-
>>  arch/riscv/net/bpf_jit.h        |   1 +
>>  arch/riscv/net/bpf_jit_comp64.c | 185 +++++++++++++++++++++++++-------
>>  arch/riscv/net/bpf_jit_core.c   |  18 +++-
>>  4 files changed, 185 insertions(+), 46 deletions(-)
>>
>> diff --git a/arch/riscv/mm/extable.c b/arch/riscv/mm/extable.c
>> index 2fc729422151..442695393131 100644
>> --- a/arch/riscv/mm/extable.c
>> +++ b/arch/riscv/mm/extable.c
>> @@ -11,14 +11,31 @@
>>  #include <linux/module.h>
>>  #include <linux/uaccess.h>
>>
>> +#ifdef CONFIG_BPF_JIT
>> +static inline bool in_bpf_jit(struct pt_regs *regs)
>> +{
>> +       if (!IS_ENABLED(CONFIG_BPF_JIT))
>> +               return false;
>
> The whole function is gated by the ifdef. No need for this check. Please remove!

ok, It should be removed here.

>
>> +
>> +       return regs->epc >= BPF_JIT_REGION_START && regs->epc < BPF_JIT_REGION_END;
>> +}
>> +
>> +int rv_bpf_fixup_exception(const struct exception_table_entry *ex, struct pt_regs *regs);
>> +#endif
>> +
>>  int fixup_exception(struct pt_regs *regs)
>>  {
>>         const struct exception_table_entry *fixup;
>>
>>         fixup = search_exception_tables(regs->epc);
>> -       if (fixup) {
>> -               regs->epc = fixup->fixup;
>> -               return 1;
>> -       }
>> -       return 0;
>> +       if (!fixup)
>> +               return 0;
>> +
>> +#ifdef CONFIG_BPF_JIT
>> +       if (in_bpf_jit(regs))
>> +               return rv_bpf_fixup_exception(fixup, regs);
>> +#endif
>> +
>> +       regs->epc = fixup->fixup;
>> +       return 1;
>>  }
>> diff --git a/arch/riscv/net/bpf_jit.h b/arch/riscv/net/bpf_jit.h
>> index 75c1e9996867..8f2e5670c1aa 100644
>> --- a/arch/riscv/net/bpf_jit.h
>> +++ b/arch/riscv/net/bpf_jit.h
>> @@ -71,6 +71,7 @@ struct rv_jit_context {
>>         int ninsns;
>>         int epilogue_offset;
>>         int *offset;            /* BPF to RV */
>> +       int nexentrys;
>
> Nit: Spelling: entries, not entrys.

typo.

>
>>         unsigned long flags;
>>         int stack_size;
>>  };
>> diff --git a/arch/riscv/net/bpf_jit_comp64.c b/arch/riscv/net/bpf_jit_comp64.c
>> index 3af4131c22c7..a1b9fe14ead3 100644
>> --- a/arch/riscv/net/bpf_jit_comp64.c
>> +++ b/arch/riscv/net/bpf_jit_comp64.c
>> @@ -5,6 +5,7 @@
>>   *
>>   */
>>
>> +#include <linux/bitfield.h>
>>  #include <linux/bpf.h>
>>  #include <linux/filter.h>
>>  #include "bpf_jit.h"
>> @@ -27,6 +28,21 @@ static const int regmap[] = {
>>         [BPF_REG_AX] =  RV_REG_T0,
>>  };
>>
>> +static const int pt_regmap[] = {
>> +       [RV_REG_A5] = offsetof(struct pt_regs, a5),
>
> Nit: Please place the A5 *under* A4.

ok, A5 should be placed under A4.
>
>> +       [RV_REG_A0] = offsetof(struct pt_regs, a0),
>> +       [RV_REG_A1] = offsetof(struct pt_regs, a1),
>> +       [RV_REG_A2] = offsetof(struct pt_regs, a2),
>> +       [RV_REG_A3] = offsetof(struct pt_regs, a3),
>> +       [RV_REG_A4] = offsetof(struct pt_regs, a4),
>> +       [RV_REG_S1] = offsetof(struct pt_regs, s1),
>> +       [RV_REG_S2] = offsetof(struct pt_regs, s2),
>> +       [RV_REG_S3] = offsetof(struct pt_regs, s3),
>> +       [RV_REG_S4] = offsetof(struct pt_regs, s4),
>> +       [RV_REG_S5] = offsetof(struct pt_regs, s5),
>> +       [RV_REG_T0] = offsetof(struct pt_regs, t0),
>> +};
>> +
>>  enum {
>>         RV_CTX_F_SEEN_TAIL_CALL =       0,
>>         RV_CTX_F_SEEN_CALL =            RV_REG_RA,
>> @@ -440,6 +456,69 @@ static int emit_call(bool fixed, u64 addr, struct rv_jit_context *ctx)
>>         return 0;
>>  }
>>
>> +#define BPF_FIXUP_OFFSET_MASK   GENMASK(26, 0)
>> +#define BPF_FIXUP_REG_MASK      GENMASK(31, 27)
>> +
>> +int rv_bpf_fixup_exception(const struct exception_table_entry *ex,
>> +                               struct pt_regs *regs)
>> +{
>> +       off_t offset = FIELD_GET(BPF_FIXUP_OFFSET_MASK, ex->fixup);
>> +       int regs_offset = FIELD_GET(BPF_FIXUP_REG_MASK, ex->fixup);
>> +
>> +       *(unsigned long *)((unsigned char *)regs + pt_regmap[regs_offset]) = 0;
>
> Nit: Inconsistency. Sometimes you use (void *) cast for byte access,
> sometimes (unsigned char *).  I'd change it to void * here, and keep
> the (void *) below.

Ummm, It is common to use void* for transit, such as container_of.

>
>> +       regs->epc = (unsigned long)&ex->fixup - offset;
>> +
>> +       return 1;
>> +}
>> +
>> +/* For accesses to BTF pointers, add an entry to the exception table */
>> +static int add_exception_handler(const struct bpf_insn *insn,
>> +                                struct rv_jit_context *ctx,
>> +                                int dst_reg, int insn_len)
>> +{
>> +       struct exception_table_entry *ex;
>> +       unsigned long pc;
>> +       off_t offset;
>> +
>> +       if (!ctx->insns || !ctx->prog->aux->extable || BPF_MODE(insn->code) != BPF_PROBE_MEM)
>> +               return 0;
>> +
>> +       if (WARN_ON_ONCE(ctx->nexentrys >= ctx->prog->aux->num_exentries))
>> +               return -EINVAL;
>> +
>> +       if (WARN_ON_ONCE(insn_len > ctx->ninsns))
>> +               return -EINVAL;
>> +
>> +       if (WARN_ON_ONCE(!rvc_enabled() && insn_len == 1))
>> +               return -EINVAL;
>> +
>> +       ex = &ctx->prog->aux->extable[ctx->nexentrys];
>> +       pc = (unsigned long)&ctx->insns[ctx->ninsns - insn_len];
>> +
>> +       offset = pc - (long)&ex->insn;
>> +       if (WARN_ON_ONCE(offset >= 0 || offset < INT_MIN))
>> +               return -ERANGE;
>> +       ex->insn = pc;
>> +
>> +       /*
>> +        * Since the extable follows the program, the fixup offset is always
>> +        * negative and limited to BPF_JIT_REGION_SIZE. Store a positive value
>> +        * to keep things simple, and put the destination register in the upper
>> +        * bits. We don't need to worry about buildtime or runtime sort
>> +        * modifying the upper bits because the table is already sorted, and
>> +        * isn't part of the main exception table.
>> +        */
>> +       offset = (long)&ex->fixup - (pc + insn_len * sizeof(u16));
>> +       if (!FIELD_FIT(BPF_FIXUP_OFFSET_MASK, offset))
>> +               return -ERANGE;
>> +
>> +       ex->fixup = FIELD_PREP(BPF_FIXUP_OFFSET_MASK, offset) |
>> +               FIELD_PREP(BPF_FIXUP_REG_MASK, dst_reg);
>> +
>> +       ctx->nexentrys++;
>> +       return 0;
>> +}
>> +
>>  int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
>>                       bool extra_pass)
>>  {
>> @@ -893,52 +972,86 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
>>
>>         /* LDX: dst = *(size *)(src + off) */
>>         case BPF_LDX | BPF_MEM | BPF_B:
>> -               if (is_12b_int(off)) {
>> -                       emit(rv_lbu(rd, off, rs), ctx);
>> +       case BPF_LDX | BPF_MEM | BPF_H:
>> +       case BPF_LDX | BPF_MEM | BPF_W:
>> +       case BPF_LDX | BPF_MEM | BPF_DW:
>> +       case BPF_LDX | BPF_PROBE_MEM | BPF_B:
>> +       case BPF_LDX | BPF_PROBE_MEM | BPF_H:
>> +       case BPF_LDX | BPF_PROBE_MEM | BPF_W:
>> +       case BPF_LDX | BPF_PROBE_MEM | BPF_DW:
>> +       {
>> +               int insn_len, insns_start;
>> +
>> +               switch (BPF_SIZE(code)) {
>> +               case BPF_B:
>> +                       if (is_12b_int(off)) {
>> +                               insns_start = ctx->ninsns;
>> +                               emit(rv_lbu(rd, off, rs), ctx);
>> +                               insn_len = ctx->ninsns - insns_start;
>> +                               break;
>> +                       }
>> +
>> +                       emit_imm(RV_REG_T1, off, ctx);
>> +                       emit_add(RV_REG_T1, RV_REG_T1, rs, ctx);
>> +                       insns_start = ctx->ninsns;
>> +                       emit(rv_lbu(rd, 0, RV_REG_T1), ctx);
>> +                       insn_len = ctx->ninsns - insns_start;
>> +                       if (insn_is_zext(&insn[1]))
>> +                               return 1;
>>                         break;
>> -               }
>> +               case BPF_H:
>> +                       if (is_12b_int(off)) {
>> +                               insns_start = ctx->ninsns;
>> +                               emit(rv_lhu(rd, off, rs), ctx);
>> +                               insn_len = ctx->ninsns - insns_start;
>> +                               break;
>> +                       }
>>
>> -               emit_imm(RV_REG_T1, off, ctx);
>> -               emit_add(RV_REG_T1, RV_REG_T1, rs, ctx);
>> -               emit(rv_lbu(rd, 0, RV_REG_T1), ctx);
>> -               if (insn_is_zext(&insn[1]))
>> -                       return 1;
>> -               break;
>> -       case BPF_LDX | BPF_MEM | BPF_H:
>> -               if (is_12b_int(off)) {
>> -                       emit(rv_lhu(rd, off, rs), ctx);
>> +                       emit_imm(RV_REG_T1, off, ctx);
>> +                       emit_add(RV_REG_T1, RV_REG_T1, rs, ctx);
>> +                       insns_start = ctx->ninsns;
>> +                       emit(rv_lhu(rd, 0, RV_REG_T1), ctx);
>> +                       insn_len = ctx->ninsns - insns_start;
>> +                       if (insn_is_zext(&insn[1]))
>> +                               return 1;
>>                         break;
>> -               }
>> +               case BPF_W:
>> +                       if (is_12b_int(off)) {
>> +                               insns_start = ctx->ninsns;
>> +                               emit(rv_lwu(rd, off, rs), ctx);
>> +                               insn_len = ctx->ninsns - insns_start;
>> +                               break;
>> +                       }
>>
>> -               emit_imm(RV_REG_T1, off, ctx);
>> -               emit_add(RV_REG_T1, RV_REG_T1, rs, ctx);
>> -               emit(rv_lhu(rd, 0, RV_REG_T1), ctx);
>> -               if (insn_is_zext(&insn[1]))
>> -                       return 1;
>> -               break;
>> -       case BPF_LDX | BPF_MEM | BPF_W:
>> -               if (is_12b_int(off)) {
>> -                       emit(rv_lwu(rd, off, rs), ctx);
>> +                       emit_imm(RV_REG_T1, off, ctx);
>> +                       emit_add(RV_REG_T1, RV_REG_T1, rs, ctx);
>> +                       insns_start = ctx->ninsns;
>> +                       emit(rv_lwu(rd, 0, RV_REG_T1), ctx);
>> +                       insn_len = ctx->ninsns - insns_start;
>> +                       if (insn_is_zext(&insn[1]))
>> +                               return 1;
>>                         break;
>> -               }
>> +               case BPF_DW:
>> +                       if (is_12b_int(off)) {
>> +                               insns_start = ctx->ninsns;
>> +                               emit_ld(rd, off, rs, ctx);
>> +                               insn_len = ctx->ninsns - insns_start;
>> +                               break;
>> +                       }
>>
>> -               emit_imm(RV_REG_T1, off, ctx);
>> -               emit_add(RV_REG_T1, RV_REG_T1, rs, ctx);
>> -               emit(rv_lwu(rd, 0, RV_REG_T1), ctx);
>> -               if (insn_is_zext(&insn[1]))
>> -                       return 1;
>> -               break;
>> -       case BPF_LDX | BPF_MEM | BPF_DW:
>> -               if (is_12b_int(off)) {
>> -                       emit_ld(rd, off, rs, ctx);
>> +                       emit_imm(RV_REG_T1, off, ctx);
>> +                       emit_add(RV_REG_T1, RV_REG_T1, rs, ctx);
>> +                       insns_start = ctx->ninsns;
>> +                       emit_ld(rd, 0, RV_REG_T1, ctx);
>> +                       insn_len = ctx->ninsns - insns_start;
>>                         break;
>>                 }
>>
>> -               emit_imm(RV_REG_T1, off, ctx);
>> -               emit_add(RV_REG_T1, RV_REG_T1, rs, ctx);
>> -               emit_ld(rd, 0, RV_REG_T1, ctx);
>> +               ret = add_exception_handler(insn, ctx, rd, insn_len);
>> +               if (ret)
>> +                       return ret;
>>                 break;
>> -
>> +       }
>>         /* speculation barrier */
>>         case BPF_ST | BPF_NOSPEC:
>>                 break;
>> diff --git a/arch/riscv/net/bpf_jit_core.c b/arch/riscv/net/bpf_jit_core.c
>> index fed86f42dfbe..5f2a842ec6f3 100644
>> --- a/arch/riscv/net/bpf_jit_core.c
>> +++ b/arch/riscv/net/bpf_jit_core.c
>> @@ -41,12 +41,12 @@ bool bpf_jit_needs_zext(void)
>>
>>  struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
>>  {
>> +       unsigned int image_size, prog_size, extable_size;
>>         bool tmp_blinded = false, extra_pass = false;
>>         struct bpf_prog *tmp, *orig_prog = prog;
>>         int pass = 0, prev_ninsns = 0, i;
>>         struct rv_jit_data *jit_data;
>>         struct rv_jit_context *ctx;
>> -       unsigned int image_size = 0;
>
> Hmm, image_size is now the *program size* plus the extable. So,
> prog_size is what image_size was. If my memory is not failing I
> *think* that the image_size has to be initialized to zero , if so this
> new prog_size has to be initialized to zero. I might be wrong. I just
> want to make sure that we're not introducing uninitialized data
> access.
>
> Same question for the extable_size. I see it's being used outside the
> for-loop below.
>
> To me it looks like both prog_size and extable_size needs to be
> initialized to zero.

I checked the logic. indeed, prog_size and extable_size needs to be initialized to 0.
>
>>
>>         if (!prog->jit_requested)
>>                 return orig_prog;
>> @@ -73,7 +73,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
>>
>>         if (ctx->offset) {
>>                 extra_pass = true;
>> -               image_size = sizeof(*ctx->insns) * ctx->ninsns;
>> +               prog_size = sizeof(*ctx->insns) * ctx->ninsns;
>>                 goto skip_init_ctx;
>>         }
>>
>> @@ -102,8 +102,12 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
>>                 if (ctx->ninsns == prev_ninsns) {
>>                         if (jit_data->header)
>>                                 break;
>> +                       /* obtain the actual image size */
>> +                       extable_size = prog->aux->num_exentries *
>> +                               sizeof(struct exception_table_entry);
>> +                       prog_size = sizeof(*ctx->insns) * ctx->ninsns;
>> +                       image_size = prog_size + extable_size;
>
> image_size is only used in the call to bpf_jit_binary_alloc(). I'd
> remove it and only use prog_size + extable_size in the call. Or move
> it into the if-statement.
>
ok, remove image_size to simplified code.

>>
>> -                       image_size = sizeof(*ctx->insns) * ctx->ninsns;
>>                         jit_data->header =
>>                                 bpf_jit_binary_alloc(image_size,
>>                                                      &jit_data->image,
>> @@ -130,9 +134,13 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
>>                 goto out_offset;
>>         }
>>
>> +       if (extable_size)
>> +               prog->aux->extable = (void *)ctx->insns + prog_size;
>
> (This was the void*-cast I was talking about)
>
>
>> skip_init_ctx:
>>         pass++;
>>         ctx->ninsns = 0;
>> +       ctx->nexentrys = 0;
>>
>>         bpf_jit_build_prologue(ctx);
>>         if (build_body(ctx, extra_pass, NULL)) {
>> @@ -143,11 +151,11 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
>>         bpf_jit_build_epilogue(ctx);
>>
>>         if (bpf_jit_enable > 1)
>> -               bpf_jit_dump(prog->len, image_size, pass, ctx->insns);
>> +               bpf_jit_dump(prog->len, prog_size, pass, ctx->insns);
>>
>>         prog->bpf_func = (void *)ctx->insns;
>>         prog->jited = 1;
>> -       prog->jited_len = image_size;
>> +       prog->jited_len = prog_size;
>>
>>         bpf_flush_icache(jit_data->header, ctx->insns + ctx->ninsns);
>>
>> --
>> 2.25.1
>
>
>
> Again, thank you for hacking on this!

thanks.
>
>
> Cheers,
> Björn
> .
>