Re: [PATCH bpf-next v2 1/2] bpf,riscv: Implement PROBE_MEM32 pseudo instructions

From: Björn Töpel
Date: Mon Mar 25 2024 - 13:44:55 EST

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In reply to: Puranjay Mohan: "[PATCH bpf-next v2 1/2] bpf,riscv: Implement PROBE_MEM32 pseudo instructions"
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Puranjay Mohan <puranjay12@xxxxxxxxx> writes:

> Add support for [LDX | STX | ST], PROBE_MEM32, [B | H | W | DW]
> instructions. They are similar to PROBE_MEM instructions with the
> following differences:
> - PROBE_MEM32 supports store.
> - PROBE_MEM32 relies on the verifier to clear upper 32-bit of the
> src/dst register
> - PROBE_MEM32 adds 64-bit kern_vm_start address (which is stored in S7
> in the prologue). Due to bpf_arena constructions such S7 + reg +
> off16 access is guaranteed to be within arena virtual range, so no
> address check at run-time.
> - S7 is a free callee-saved register, so it is used to store kern_vm_start
> - PROBE_MEM32 allows STX and ST. If they fault the store is a nop. When
> LDX faults the destination register is zeroed.
>
> To support these on riscv, we do tmp = S7 + src/dst reg and then use
> tmp2 as the new src/dst register. This allows us to reuse most of the
> code for normal [LDX | STX | ST].

Cool to see the RV BPF JIT keeping up with x86 features! ;-) Nice work!

A couple of minor comments below.

> Signed-off-by: Puranjay Mohan <puranjay12@xxxxxxxxx>
> ---
> arch/riscv/net/bpf_jit.h | 1 +
> arch/riscv/net/bpf_jit_comp64.c | 193 +++++++++++++++++++++++++++++++-
> arch/riscv/net/bpf_jit_core.c | 1 +
> 3 files changed, 192 insertions(+), 3 deletions(-)
>
> diff --git a/arch/riscv/net/bpf_jit.h b/arch/riscv/net/bpf_jit.h
> index f4b6b3b9edda..8a47da08dd9c 100644
> --- a/arch/riscv/net/bpf_jit.h
> +++ b/arch/riscv/net/bpf_jit.h
> @@ -81,6 +81,7 @@ struct rv_jit_context {
> int nexentries;
> unsigned long flags;
> int stack_size;
> + u64 arena_vm_start;
> };
>
> /* Convert from ninsns to bytes. */
> diff --git a/arch/riscv/net/bpf_jit_comp64.c b/arch/riscv/net/bpf_jit_comp64.c
> index 1adf2f39ce59..0c0588e327af 100644
> --- a/arch/riscv/net/bpf_jit_comp64.c
> +++ b/arch/riscv/net/bpf_jit_comp64.c
> @@ -255,6 +255,10 @@ static void __build_epilogue(bool is_tail_call, struct rv_jit_context *ctx)
> emit_ld(RV_REG_S6, store_offset, RV_REG_SP, ctx);
> store_offset -= 8;
> }
> + if (ctx->arena_vm_start) {
> + emit_ld(RV_REG_S7, store_offset, RV_REG_SP, ctx);
> + store_offset -= 8;
> + }
>
> emit_addi(RV_REG_SP, RV_REG_SP, stack_adjust, ctx);
> /* Set return value. */
> @@ -548,6 +552,7 @@ static void emit_atomic(u8 rd, u8 rs, s16 off, s32 imm, bool is64,
>
> #define BPF_FIXUP_OFFSET_MASK GENMASK(26, 0)
> #define BPF_FIXUP_REG_MASK GENMASK(31, 27)
> +#define DONT_CLEAR 17 /* RV_REG_A7 unused in pt_regmap */

Hmm, so this is just a a sentinel node, right? Isn't it more robust to
use, say REG_ZERO which will never be used? Maybe REG_DONT_CLEAR_MARKER
or smth, so it's obvious how it's used?

> bool ex_handler_bpf(const struct exception_table_entry *ex,
> struct pt_regs *regs)
> @@ -555,7 +560,8 @@ bool ex_handler_bpf(const struct exception_table_entry *ex,
> off_t offset = FIELD_GET(BPF_FIXUP_OFFSET_MASK, ex->fixup);
> int regs_offset = FIELD_GET(BPF_FIXUP_REG_MASK, ex->fixup);
>
> - *(unsigned long *)((void *)regs + pt_regmap[regs_offset]) = 0;
> + if (regs_offset != DONT_CLEAR)
> + *(unsigned long *)((void *)regs + pt_regmap[regs_offset]) = 0;
> regs->epc = (unsigned long)&ex->fixup - offset;
>
> return true;
> @@ -572,7 +578,8 @@ static int add_exception_handler(const struct bpf_insn *insn,
> off_t fixup_offset;
>
> if (!ctx->insns || !ctx->ro_insns || !ctx->prog->aux->extable ||
> - (BPF_MODE(insn->code) != BPF_PROBE_MEM && BPF_MODE(insn->code) != BPF_PROBE_MEMSX))
> + (BPF_MODE(insn->code) != BPF_PROBE_MEM && BPF_MODE(insn->code) != BPF_PROBE_MEMSX &&
> + BPF_MODE(insn->code) != BPF_PROBE_MEM32))
> return 0;
>
> if (WARN_ON_ONCE(ctx->nexentries >= ctx->prog->aux->num_exentries))
> @@ -622,6 +629,9 @@ static int add_exception_handler(const struct bpf_insn *insn,
>
> ex->insn = ins_offset;
>
> + if (BPF_CLASS(insn->code) != BPF_LDX)
> + dst_reg = DONT_CLEAR;
> +

Instead of having a side-effect, and passing a dummy dst_reg for the
probe_mem32, just explicitly add DONT_CLEAR when calling
add_exception_handler(). It's more obvious to me at least.

> ex->fixup = FIELD_PREP(BPF_FIXUP_OFFSET_MASK, fixup_offset) |
> FIELD_PREP(BPF_FIXUP_REG_MASK, dst_reg);
> ex->type = EX_TYPE_BPF;
> @@ -1063,7 +1073,7 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
> BPF_CLASS(insn->code) == BPF_JMP;
> int s, e, rvoff, ret, i = insn - ctx->prog->insnsi;
> struct bpf_prog_aux *aux = ctx->prog->aux;
> - u8 rd = -1, rs = -1, code = insn->code;
> + u8 rd = -1, rs = -1, code = insn->code, reg_arena_vm_start = RV_REG_S7;
> s16 off = insn->off;
> s32 imm = insn->imm;
>
> @@ -1539,6 +1549,11 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
> case BPF_LDX | BPF_PROBE_MEMSX | BPF_B:
> case BPF_LDX | BPF_PROBE_MEMSX | BPF_H:
> case BPF_LDX | BPF_PROBE_MEMSX | BPF_W:
> + /* LDX | PROBE_MEM32: dst = *(unsigned size *)(src + S7 + off)*/
> + case BPF_LDX | BPF_PROBE_MEM32 | BPF_B:
> + case BPF_LDX | BPF_PROBE_MEM32 | BPF_H:
> + case BPF_LDX | BPF_PROBE_MEM32 | BPF_W:
> + case BPF_LDX | BPF_PROBE_MEM32 | BPF_DW:
> {
> int insn_len, insns_start;
> bool sign_ext;
> @@ -1546,6 +1561,11 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
> sign_ext = BPF_MODE(insn->code) == BPF_MEMSX ||
> BPF_MODE(insn->code) == BPF_PROBE_MEMSX;
>
> + if (BPF_MODE(insn->code) == BPF_PROBE_MEM32) {
> + emit_add(RV_REG_T2, rs, reg_arena_vm_start, ctx);
> + rs = RV_REG_T2;
> + }
> +
> switch (BPF_SIZE(code)) {
> case BPF_B:
> if (is_12b_int(off)) {
> @@ -1682,6 +1702,87 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
> emit_sd(RV_REG_T2, 0, RV_REG_T1, ctx);
> break;
>
> + case BPF_ST | BPF_PROBE_MEM32 | BPF_B:
> + case BPF_ST | BPF_PROBE_MEM32 | BPF_H:
> + case BPF_ST | BPF_PROBE_MEM32 | BPF_W:
> + case BPF_ST | BPF_PROBE_MEM32 | BPF_DW:
> + {
> + int insn_len, insns_start;
> +
> + emit_add(RV_REG_T3, rd, reg_arena_vm_start, ctx);
> + rd = RV_REG_T3;
> +
> + /* Load imm to a register then store it */
> + emit_imm(RV_REG_T1, imm, ctx);
> +
> + switch (BPF_SIZE(code)) {
> + case BPF_B:
> + if (is_12b_int(off)) {
> + insns_start = ctx->ninsns;
> + emit(rv_sb(rd, off, RV_REG_T1), ctx);
> + insn_len = ctx->ninsns - insns_start;
> + break;
> + }
> +
> + emit_imm(RV_REG_T2, off, ctx);
> + emit_add(RV_REG_T2, RV_REG_T2, rd, ctx);
> + insns_start = ctx->ninsns;
> + emit(rv_sb(RV_REG_T2, 0, RV_REG_T1), ctx);
> + insn_len = ctx->ninsns - insns_start;
> +
> + break;
> +
> + case BPF_H:
> + if (is_12b_int(off)) {
> + insns_start = ctx->ninsns;
> + emit(rv_sh(rd, off, RV_REG_T1), ctx);
> + insn_len = ctx->ninsns - insns_start;
> + break;
> + }
> +
> + emit_imm(RV_REG_T2, off, ctx);
> + emit_add(RV_REG_T2, RV_REG_T2, rd, ctx);
> + insns_start = ctx->ninsns;
> + emit(rv_sh(RV_REG_T2, 0, RV_REG_T1), ctx);
> + insn_len = ctx->ninsns - insns_start;
> + break;
> + case BPF_W:
> + if (is_12b_int(off)) {
> + insns_start = ctx->ninsns;
> + emit_sw(rd, off, RV_REG_T1, ctx);
> + insn_len = ctx->ninsns - insns_start;
> + break;
> + }
> +
> + emit_imm(RV_REG_T2, off, ctx);
> + emit_add(RV_REG_T2, RV_REG_T2, rd, ctx);
> + insns_start = ctx->ninsns;
> + emit_sw(RV_REG_T2, 0, RV_REG_T1, ctx);
> + insn_len = ctx->ninsns - insns_start;
> + break;
> + case BPF_DW:
> + if (is_12b_int(off)) {
> + insns_start = ctx->ninsns;
> + emit_sd(rd, off, RV_REG_T1, ctx);
> + insn_len = ctx->ninsns - insns_start;
> + break;
> + }
> +
> + emit_imm(RV_REG_T2, off, ctx);
> + emit_add(RV_REG_T2, RV_REG_T2, rd, ctx);
> + insns_start = ctx->ninsns;
> + emit_sd(RV_REG_T2, 0, RV_REG_T1, ctx);
> + insn_len = ctx->ninsns - insns_start;
> + break;
> + }

A lot of similar code, with emit of different sizes. Possible to move
move out to a function, and wrap the emits? The main loop is hard read
already!

> +
> + ret = add_exception_handler(insn, ctx, rd, insn_len);
> + if (ret)
> + return ret;
> +
> + break;
> + }
> +
> /* STX: *(size *)(dst + off) = src */
> case BPF_STX | BPF_MEM | BPF_B:
> if (is_12b_int(off)) {
> @@ -1728,6 +1829,83 @@ int bpf_jit_emit_insn(const struct bpf_insn *insn, struct rv_jit_context *ctx,
> emit_atomic(rd, rs, off, imm,
> BPF_SIZE(code) == BPF_DW, ctx);
> break;
> +
> + case BPF_STX | BPF_PROBE_MEM32 | BPF_B:
> + case BPF_STX | BPF_PROBE_MEM32 | BPF_H:
> + case BPF_STX | BPF_PROBE_MEM32 | BPF_W:
> + case BPF_STX | BPF_PROBE_MEM32 | BPF_DW:
> + {
> + int insn_len, insns_start;
> +
> + emit_add(RV_REG_T2, rd, reg_arena_vm_start, ctx);
> + rd = RV_REG_T2;
> +
> + switch (BPF_SIZE(code)) {
> + case BPF_B:
> + if (is_12b_int(off)) {
> + insns_start = ctx->ninsns;
> + emit(rv_sb(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, rd, ctx);
> + insns_start = ctx->ninsns;
> + emit(rv_sb(RV_REG_T1, 0, rs), ctx);
> + insn_len = ctx->ninsns - insns_start;
> + break;
> + case BPF_H:
> + if (is_12b_int(off)) {
> + insns_start = ctx->ninsns;
> + emit(rv_sh(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, rd, ctx);
> + insns_start = ctx->ninsns;
> + emit(rv_sh(RV_REG_T1, 0, rs), ctx);
> + insn_len = ctx->ninsns - insns_start;
> + break;
> + case BPF_W:
> + if (is_12b_int(off)) {
> + insns_start = ctx->ninsns;
> + emit_sw(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, rd, ctx);
> + insns_start = ctx->ninsns;
> + emit_sw(RV_REG_T1, 0, rs, ctx);
> + insn_len = ctx->ninsns - insns_start;
> + break;
> + case BPF_DW:
> + if (is_12b_int(off)) {
> + insns_start = ctx->ninsns;
> + emit_sd(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, rd, ctx);
> + insns_start = ctx->ninsns;
> + emit_sd(RV_REG_T1, 0, rs, ctx);
> + insn_len = ctx->ninsns - insns_start;
> + break;
> + }

Same comment as above.

Björn

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