[PATCH 7/7] LoongArch: bpf: Add BPF exception tables

From: Youling Tang
Date: Mon Oct 10 2022 - 05:35:15 EST

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Inspired by commit 800834285361("bpf, arm64: Add BPF exception tables"),
do similar to LoongArch to add BPF exception tables.

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 LoongArch 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.

To keep the compact exception table entry format, inspect the pc in
fixup_exception(). A more generic solution would add a "handler" field
to the table entry, like on x86 and arm64 etc.

Signed-off-by: Youling Tang <tangyouling@xxxxxxxxxxx>
---
arch/loongarch/include/asm/asm-extable.h | 1 +
arch/loongarch/include/asm/extable.h | 12 ++++
arch/loongarch/mm/extable.c | 2 +
arch/loongarch/net/bpf_jit.c | 84 ++++++++++++++++++++++--
arch/loongarch/net/bpf_jit.h | 2 +
5 files changed, 96 insertions(+), 5 deletions(-)

diff --git a/arch/loongarch/include/asm/asm-extable.h b/arch/loongarch/include/asm/asm-extable.h
index f5502cb50c6e..df05005f2b80 100644
--- a/arch/loongarch/include/asm/asm-extable.h
+++ b/arch/loongarch/include/asm/asm-extable.h
@@ -5,6 +5,7 @@
#define EX_TYPE_NONE 0
#define EX_TYPE_FIXUP 1
#define EX_TYPE_UACCESS_ERR_ZERO 2
+#define EX_TYPE_BPF 3

#ifdef __ASSEMBLY__

diff --git a/arch/loongarch/include/asm/extable.h b/arch/loongarch/include/asm/extable.h
index 92612b4364a1..9f15f545e664 100644
--- a/arch/loongarch/include/asm/extable.h
+++ b/arch/loongarch/include/asm/extable.h
@@ -32,6 +32,18 @@ do { \
(b)->data = (tmp).data; \
} while (0)

+#ifdef CONFIG_BPF_JIT
+bool ex_handler_bpf(const struct exception_table_entry *ex,
+ struct pt_regs *regs);
+#else
+static inline
+bool ex_handler_bpf(const struct exception_table_entry *ex,
+ struct pt_regs *regs)
+{
+ return false;
+}
+#endif /* !CONFIG_BPF_JIT */
+
bool fixup_exception(struct pt_regs *regs);

#endif
diff --git a/arch/loongarch/mm/extable.c b/arch/loongarch/mm/extable.c
index e6871edd0631..3bb90c705b1b 100644
--- a/arch/loongarch/mm/extable.c
+++ b/arch/loongarch/mm/extable.c
@@ -58,6 +58,8 @@ bool fixup_exception(struct pt_regs *regs)
return ex_handler_fixup(ex, regs);
case EX_TYPE_UACCESS_ERR_ZERO:
return ex_handler_uaccess_err_zero(ex, regs);
+ case EX_TYPE_BPF:
+ return ex_handler_bpf(ex, regs);
}

BUG();
diff --git a/arch/loongarch/net/bpf_jit.c b/arch/loongarch/net/bpf_jit.c
index 9963ae69b2aa..ce96d38000e4 100644
--- a/arch/loongarch/net/bpf_jit.c
+++ b/arch/loongarch/net/bpf_jit.c
@@ -388,6 +388,63 @@ static bool is_signed_bpf_cond(u8 cond)
cond == BPF_JSGE || cond == BPF_JSLE;
}

+#define BPF_FIXUP_OFFSET_MASK GENMASK(26, 0)
+#define BPF_FIXUP_REG_MASK GENMASK(31, 27)
+
+bool ex_handler_bpf(const struct exception_table_entry *ex,
+ struct pt_regs *regs)
+{
+ off_t offset = FIELD_GET(BPF_FIXUP_OFFSET_MASK, ex->fixup);
+ int dst_reg = FIELD_GET(BPF_FIXUP_REG_MASK, ex->fixup);
+
+ regs->regs[dst_reg] = 0;
+ regs->csr_era = (unsigned long)&ex->fixup - offset;
+ return true;
+}
+
+/* For accesses to BTF pointers, add an entry to the exception table */
+static int add_exception_handler(const struct bpf_insn *insn,
+ struct jit_ctx *ctx,
+ int dst_reg)
+{
+ struct exception_table_entry *ex;
+ unsigned long pc;
+ off_t offset;
+
+ if (!ctx->image || !ctx->prog->aux->extable || BPF_MODE(insn->code) != BPF_PROBE_MEM)
+ return 0;
+
+ if (WARN_ON_ONCE(ctx->num_exentries >= ctx->prog->aux->num_exentries))
+ return -EINVAL;
+
+ ex = &ctx->prog->aux->extable[ctx->num_exentries];
+ pc = (unsigned long)&ctx->image[ctx->idx - 1];
+
+ offset = pc - (long)&ex->insn;
+ if (WARN_ON_ONCE(offset >= 0 || offset < INT_MIN))
+ return -ERANGE;
+ ex->insn = offset;
+
+ /*
+ * 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 + LOONGARCH_INSN_SIZE);
+ 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);
+ ex->type = EX_TYPE_BPF;
+
+ ctx->num_exentries++;
+ return 0;
+}
+
static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx, bool extra_pass)
{
const bool is32 = BPF_CLASS(insn->code) == BPF_ALU ||
@@ -821,6 +878,10 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx, bool ext
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_DW:
+ case BPF_LDX | BPF_PROBE_MEM | BPF_W:
+ case BPF_LDX | BPF_PROBE_MEM | BPF_H:
+ case BPF_LDX | BPF_PROBE_MEM | BPF_B:
switch (BPF_SIZE(code)) {
case BPF_B:
if (is_signed_imm12(off)) {
@@ -859,6 +920,10 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx, bool ext
}
break;
}
+
+ ret = add_exception_handler(insn, ctx, dst);
+ if (ret)
+ return ret;
break;

/* *(size *)(dst + off) = imm */
@@ -1024,6 +1089,9 @@ static int validate_code(struct jit_ctx *ctx)
return -1;
}

+ if (WARN_ON_ONCE(ctx->num_exentries != ctx->prog->aux->num_exentries))
+ return -1;
+
return 0;
}

@@ -1031,7 +1099,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
{
bool tmp_blinded = false, extra_pass = false;
u8 *image_ptr;
- int image_size;
+ int image_size, prog_size, extable_size;
struct jit_ctx ctx;
struct jit_data *jit_data;
struct bpf_binary_header *header;
@@ -1072,7 +1140,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
image_ptr = jit_data->image;
header = jit_data->header;
extra_pass = true;
- image_size = sizeof(u32) * ctx.idx;
+ prog_size = sizeof(u32) * ctx.idx;
goto skip_init_ctx;
}

@@ -1094,12 +1162,15 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
ctx.epilogue_offset = ctx.idx;
build_epilogue(&ctx);

+ extable_size = prog->aux->num_exentries * sizeof(struct exception_table_entry);
+
/* Now we know the actual image size.
* As each LoongArch instruction is of length 32bit,
* we are translating number of JITed intructions into
* the size required to store these JITed code.
*/
- image_size = sizeof(u32) * ctx.idx;
+ prog_size = sizeof(u32) * ctx.idx;
+ image_size = prog_size + extable_size;
/* Now we know the size of the structure to make */
header = bpf_jit_binary_alloc(image_size, &image_ptr,
sizeof(u32), jit_fill_hole);
@@ -1110,9 +1181,12 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)

/* 2. Now, the actual pass to generate final JIT code */
ctx.image = (union loongarch_instruction *)image_ptr;
+ if (extable_size)
+ prog->aux->extable = (void *)image_ptr + prog_size;

skip_init_ctx:
ctx.idx = 0;
+ ctx.num_exentries = 0;

build_prologue(&ctx);
if (build_body(&ctx, extra_pass)) {
@@ -1131,7 +1205,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)

/* And we're done */
if (bpf_jit_enable > 1)
- bpf_jit_dump(prog->len, image_size, 2, ctx.image);
+ bpf_jit_dump(prog->len, prog_size, 2, ctx.image);

/* Update the icache */
flush_icache_range((unsigned long)header, (unsigned long)(ctx.image + ctx.idx));
@@ -1153,7 +1227,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
jit_data->header = header;
}
prog->jited = 1;
- prog->jited_len = image_size;
+ prog->jited_len = prog_size;
prog->bpf_func = (void *)ctx.image;

if (!prog->is_func || extra_pass) {
diff --git a/arch/loongarch/net/bpf_jit.h b/arch/loongarch/net/bpf_jit.h
index e665ddb0aeb8..ca708024fdd3 100644
--- a/arch/loongarch/net/bpf_jit.h
+++ b/arch/loongarch/net/bpf_jit.h
@@ -4,6 +4,7 @@
*
* Copyright (C) 2022 Loongson Technology Corporation Limited
*/
+#include <linux/bitfield.h>
#include <linux/bpf.h>
#include <linux/filter.h>
#include <asm/cacheflush.h>
@@ -15,6 +16,7 @@ struct jit_ctx {
unsigned int flags;
unsigned int epilogue_offset;
u32 *offset;
+ int num_exentries;
union loongarch_instruction *image;
u32 stack_size;
};
--
2.36.1

Next message: Matti Vaittinen: "Re: [RFT PATCH v3 10/10] iio: Don't silently expect attribute types"
Previous message: Youling Tang: "[PATCH 6/7] LoongArch: vmlinux.lds.S: Remove `.fixup` section"
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