Re: [PATCH bpf-next,v2] riscv, bpf: Add BPF exception tables

From: Björn Töpel
Date: Mon Oct 25 2021 - 14:27:50 EST

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On Mon, 25 Oct 2021 at 05:38, Tong Tiangen <tongtiangen@xxxxxxxxxx> 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@xxxxxxxxxx>
> Tested-by: Pu Lehui <pulehui@xxxxxxxxxx>
> ---
> 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.

> 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!

> +
> + 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.

> 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.

> + [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.

> + 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.

>
> 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.

>
> - 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!

Cheers,
Björn

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In reply to: Tong Tiangen: "[PATCH bpf-next,v2] riscv, bpf: Add BPF exception tables"
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