[PATCH bpf-next,v3] riscv, bpf: Add BPF exception tables

From: Tong Tiangen
Date: Wed Oct 27 2021 - 07:03:38 EST

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>
---
v3:
Modify according to Björn's comments, mainly code optimization.
v2:
Modify according to Björn's comments, mainly removes redundant head files
extable.h and some code style issues.

arch/riscv/mm/extable.c | 19 +++-
arch/riscv/net/bpf_jit.h | 1 +
arch/riscv/net/bpf_jit_comp64.c | 185 +++++++++++++++++++++++++-------
arch/riscv/net/bpf_jit_core.c | 19 ++--
4 files changed, 177 insertions(+), 47 deletions(-)

diff --git a/arch/riscv/mm/extable.c b/arch/riscv/mm/extable.c
index 2fc729422151..18bf338303b6 100644
--- a/arch/riscv/mm/extable.c
+++ b/arch/riscv/mm/extable.c
@@ -11,14 +11,23 @@
#include <linux/module.h>
#include <linux/uaccess.h>

+#ifdef CONFIG_BPF_JIT
+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 (regs->epc >= BPF_JIT_REGION_START && regs->epc < BPF_JIT_REGION_END)
+ 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..f42d9cd3b64d 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 nexentries;
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..2ca345c7b0bf 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_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_A5] = offsetof(struct pt_regs, a5),
+ [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 *)((void *)regs + pt_regmap[regs_offset]) = 0;
+ 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->nexentries >= 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->nexentries];
+ 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->nexentries++;
+ 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..7ccc809f2c19 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 prog_size = 0, extable_size = 0;
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;

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;
}

@@ -102,10 +102,13 @@ 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 = sizeof(*ctx->insns) * ctx->ninsns;
jit_data->header =
- bpf_jit_binary_alloc(image_size,
+ bpf_jit_binary_alloc(prog_size + extable_size,
&jit_data->image,
sizeof(u32),
bpf_fill_ill_insns);
@@ -130,9 +133,13 @@ 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;
+
skip_init_ctx:
pass++;
ctx->ninsns = 0;
+ ctx->nexentries = 0;

bpf_jit_build_prologue(ctx);
if (build_body(ctx, extra_pass, NULL)) {
@@ -143,11 +150,11 @@ 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