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SubjectRe: [PATCH 3/4] bpf powerpc: Add BPF_PROBE_MEM support for 64bit JIT
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Le 06/07/2021 à 09:32, Ravi Bangoria a écrit :
> BPF load instruction with BPF_PROBE_MEM mode can cause a fault
> inside kernel. Append exception table for such instructions
> within BPF program.

Can you do the same for 32bit ?

>
> Unlike other archs which uses extable 'fixup' field to pass dest_reg
> and nip, BPF exception table on PowerPC follows the generic PowerPC
> exception table design, where it populates both fixup and extable
> sections witin BPF program. fixup section contains two instructions,
> first instruction clears dest_reg and 2nd jumps to next instruction
> in the BPF code. extable 'insn' field contains relative offset of
> the instruction and 'fixup' field contains relative offset of the
> fixup entry. Example layout of BPF program with extable present:
>
> +------------------+
> | |
> | |
> 0x4020 -->| ld r27,4(r3) |
> | |
> | |
> 0x40ac -->| lwz r3,0(r4) |
> | |
> | |
> |------------------|
> 0x4280 -->| xor r27,r27,r27 | \ fixup entry
> | b 0x4024 | /
> 0x4288 -->| xor r3,r3,r3 |
> | b 0x40b0 |
> |------------------|
> 0x4290 -->| insn=0xfffffd90 | \ extable entry
> | fixup=0xffffffec | /
> 0x4298 -->| insn=0xfffffe14 |
> | fixup=0xffffffec |
> +------------------+
>
> (Addresses shown here are chosen random, not real)
>
> Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
> ---
> arch/powerpc/net/bpf_jit.h | 5 ++-
> arch/powerpc/net/bpf_jit_comp.c | 25 +++++++++----
> arch/powerpc/net/bpf_jit_comp32.c | 2 +-
> arch/powerpc/net/bpf_jit_comp64.c | 60 ++++++++++++++++++++++++++++++-
> 4 files changed, 83 insertions(+), 9 deletions(-)
>
> diff --git a/arch/powerpc/net/bpf_jit.h b/arch/powerpc/net/bpf_jit.h
> index 411c63d945c7..e9408ad190d3 100644
> --- a/arch/powerpc/net/bpf_jit.h
> +++ b/arch/powerpc/net/bpf_jit.h
> @@ -141,8 +141,11 @@ struct codegen_context {
> unsigned int idx;
> unsigned int stack_size;
> int b2p[ARRAY_SIZE(b2p)];
> + unsigned int exentry_idx;
> };
>
> +#define BPF_FIXUP_LEN 8 /* Two instructions */
> +
> static inline void bpf_flush_icache(void *start, void *end)
> {
> smp_wmb(); /* smp write barrier */
> @@ -166,7 +169,7 @@ static inline void bpf_clear_seen_register(struct codegen_context *ctx, int i)
>
> void bpf_jit_emit_func_call_rel(u32 *image, struct codegen_context *ctx, u64 func);
> int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, struct codegen_context *ctx,
> - u32 *addrs);
> + u32 *addrs, int pass);
> void bpf_jit_build_prologue(u32 *image, struct codegen_context *ctx);
> void bpf_jit_build_epilogue(u32 *image, struct codegen_context *ctx);
> void bpf_jit_realloc_regs(struct codegen_context *ctx);
> diff --git a/arch/powerpc/net/bpf_jit_comp.c b/arch/powerpc/net/bpf_jit_comp.c
> index a9585e52a88d..3ebd8897cf09 100644
> --- a/arch/powerpc/net/bpf_jit_comp.c
> +++ b/arch/powerpc/net/bpf_jit_comp.c
> @@ -89,6 +89,8 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *fp)
> {
> u32 proglen;
> u32 alloclen;
> + u32 extable_len = 0;
> + u32 fixup_len = 0;

Setting those to 0 doesn't seem to be needed, as it doesn't seem to exist any path to skip the
setting below. You should not perform unnecessary init at declaration as it is error prone.

> u8 *image = NULL;
> u32 *code_base;
> u32 *addrs;
> @@ -131,7 +133,6 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *fp)
> image = jit_data->image;
> bpf_hdr = jit_data->header;
> proglen = jit_data->proglen;
> - alloclen = proglen + FUNCTION_DESCR_SIZE;
> extra_pass = true;
> goto skip_init_ctx;
> }
> @@ -149,7 +150,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *fp)
> cgctx.stack_size = round_up(fp->aux->stack_depth, 16);
>
> /* Scouting faux-generate pass 0 */
> - if (bpf_jit_build_body(fp, 0, &cgctx, addrs)) {
> + if (bpf_jit_build_body(fp, 0, &cgctx, addrs, 0)) {
> /* We hit something illegal or unsupported. */
> fp = org_fp;
> goto out_addrs;
> @@ -162,7 +163,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *fp)
> */
> if (cgctx.seen & SEEN_TAILCALL) {
> cgctx.idx = 0;
> - if (bpf_jit_build_body(fp, 0, &cgctx, addrs)) {
> + if (bpf_jit_build_body(fp, 0, &cgctx, addrs, 0)) {
> fp = org_fp;
> goto out_addrs;
> }
> @@ -177,8 +178,11 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *fp)
> bpf_jit_build_prologue(0, &cgctx);
> bpf_jit_build_epilogue(0, &cgctx);
>
> + fixup_len = fp->aux->num_exentries * BPF_FIXUP_LEN;
> + extable_len = fp->aux->num_exentries * sizeof(struct exception_table_entry);
> +
> proglen = cgctx.idx * 4;
> - alloclen = proglen + FUNCTION_DESCR_SIZE;
> + alloclen = proglen + FUNCTION_DESCR_SIZE + fixup_len + extable_len;
>
> bpf_hdr = bpf_jit_binary_alloc(alloclen, &image, 4, bpf_jit_fill_ill_insns);
> if (!bpf_hdr) {
> @@ -186,6 +190,11 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *fp)
> goto out_addrs;
> }
>
> + if (extable_len) {
> + fp->aux->extable = (void *)image + FUNCTION_DESCR_SIZE +
> + proglen + fixup_len;
> + }
> +
> skip_init_ctx:
> code_base = (u32 *)(image + FUNCTION_DESCR_SIZE);
>
> @@ -210,7 +219,11 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *fp)
> /* Now build the prologue, body code & epilogue for real. */
> cgctx.idx = 0;
> bpf_jit_build_prologue(code_base, &cgctx);
> - bpf_jit_build_body(fp, code_base, &cgctx, addrs);
> + if (bpf_jit_build_body(fp, code_base, &cgctx, addrs, pass)) {
> + bpf_jit_binary_free(bpf_hdr);
> + fp = org_fp;
> + goto out_addrs;
> + }
> bpf_jit_build_epilogue(code_base, &cgctx);
>
> if (bpf_jit_enable > 1)
> @@ -234,7 +247,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *fp)
>
> fp->bpf_func = (void *)image;
> fp->jited = 1;
> - fp->jited_len = alloclen;
> + fp->jited_len = proglen + FUNCTION_DESCR_SIZE;
>
> bpf_flush_icache(bpf_hdr, (u8 *)bpf_hdr + (bpf_hdr->pages * PAGE_SIZE));
> if (!fp->is_func || extra_pass) {

This hunk does not apply on latest powerpc tree. You are missing commit 62e3d4210ac9c


> diff --git a/arch/powerpc/net/bpf_jit_comp32.c b/arch/powerpc/net/bpf_jit_comp32.c
> index 1f81bea35aab..23ab5620a45a 100644
> --- a/arch/powerpc/net/bpf_jit_comp32.c
> +++ b/arch/powerpc/net/bpf_jit_comp32.c
> @@ -266,7 +266,7 @@ static void bpf_jit_emit_tail_call(u32 *image, struct codegen_context *ctx, u32
>
> /* Assemble the body code between the prologue & epilogue */
> int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, struct codegen_context *ctx,
> - u32 *addrs)
> + u32 *addrs, int pass)
> {
> const struct bpf_insn *insn = fp->insnsi;
> int flen = fp->len;
> diff --git a/arch/powerpc/net/bpf_jit_comp64.c b/arch/powerpc/net/bpf_jit_comp64.c
> index 984177d9d394..1884c6dca89a 100644
> --- a/arch/powerpc/net/bpf_jit_comp64.c
> +++ b/arch/powerpc/net/bpf_jit_comp64.c
> @@ -270,9 +270,51 @@ static void bpf_jit_emit_tail_call(u32 *image, struct codegen_context *ctx, u32
> /* out: */
> }
>
> +static int add_extable_entry(struct bpf_prog *fp, u32 *image, int pass,
> + u32 code, struct codegen_context *ctx, int dst_reg)
> +{
> + off_t offset;
> + unsigned long pc;
> + struct exception_table_entry *ex;
> + u32 *fixup;
> +
> + /* Populate extable entries only in the last pass */
> + if (pass != 2 || BPF_MODE(code) != BPF_PROBE_MEM)

'code' is only used for that test, can you do the test before calling add_extable_entry() ?

> + return 0;
> +
> + if (!fp->aux->extable ||
> + WARN_ON_ONCE(ctx->exentry_idx >= fp->aux->num_exentries))
> + return -EINVAL;
> +
> + pc = (unsigned long)&image[ctx->idx - 1];

You should call this function before incrementing ctx->idx

> +
> + fixup = (void *)fp->aux->extable -
> + (fp->aux->num_exentries * BPF_FIXUP_LEN) +
> + (ctx->exentry_idx * BPF_FIXUP_LEN);
> +
> + fixup[0] = PPC_RAW_XOR(dst_reg, dst_reg, dst_reg);

Prefered way to clear a reg in according to ISA is to do 'li reg, 0'

> + fixup[1] = (PPC_INST_BRANCH |
> + (((long)(pc + 4) - (long)&fixup[1]) & 0x03fffffc));

Would be nice if we could have a PPC_RAW_BRANCH() stuff, we could do something like
PPC_RAW_BRANCH((long)(pc + 4) - (long)&fixup[1])


> +
> + ex = &fp->aux->extable[ctx->exentry_idx];
> +
> + offset = pc - (long)&ex->insn;
> + if (WARN_ON_ONCE(offset >= 0 || offset < INT_MIN))
> + return -ERANGE;
> + ex->insn = offset;
> +
> + offset = (long)fixup - (long)&ex->fixup;
> + if (WARN_ON_ONCE(offset >= 0 || offset < INT_MIN))
> + return -ERANGE;
> + ex->fixup = offset;
> +
> + ctx->exentry_idx++;
> + return 0;
> +}
> +
> /* Assemble the body code between the prologue & epilogue */
> int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, struct codegen_context *ctx,
> - u32 *addrs)
> + u32 *addrs, int pass)
> {
> const struct bpf_insn *insn = fp->insnsi;
> int flen = fp->len;
> @@ -710,25 +752,41 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, struct codegen_context *
> */
> /* dst = *(u8 *)(ul) (src + off) */
> case BPF_LDX | BPF_MEM | BPF_B:
> + case BPF_LDX | BPF_PROBE_MEM | BPF_B:

Could do:
+ case BPF_LDX | BPF_PROBE_MEM | BPF_B:
+ ret = add_extable_entry(fp, image, pass, code, ctx, dst_reg);
+ if (ret)
+ return ret;
case BPF_LDX | BPF_MEM | BPF_B:

> EMIT(PPC_RAW_LBZ(dst_reg, src_reg, off));
> if (insn_is_zext(&insn[i + 1]))
> addrs[++i] = ctx->idx * 4;
> + ret = add_extable_entry(fp, image, pass, code, ctx, dst_reg);
> + if (ret)
> + return ret;
> break;
> /* dst = *(u16 *)(ul) (src + off) */
> case BPF_LDX | BPF_MEM | BPF_H:
> + case BPF_LDX | BPF_PROBE_MEM | BPF_H:
> EMIT(PPC_RAW_LHZ(dst_reg, src_reg, off));
> if (insn_is_zext(&insn[i + 1]))
> addrs[++i] = ctx->idx * 4;
> + ret = add_extable_entry(fp, image, pass, code, ctx, dst_reg);
> + if (ret)
> + return ret;
> break;
> /* dst = *(u32 *)(ul) (src + off) */
> case BPF_LDX | BPF_MEM | BPF_W:
> + case BPF_LDX | BPF_PROBE_MEM | BPF_W:
> EMIT(PPC_RAW_LWZ(dst_reg, src_reg, off));
> if (insn_is_zext(&insn[i + 1]))
> addrs[++i] = ctx->idx * 4;
> + ret = add_extable_entry(fp, image, pass, code, ctx, dst_reg);
> + if (ret)
> + return ret;
> break;
> /* dst = *(u64 *)(ul) (src + off) */
> case BPF_LDX | BPF_MEM | BPF_DW:
> + case BPF_LDX | BPF_PROBE_MEM | BPF_DW:
> PPC_BPF_LL(dst_reg, src_reg, off);
> + ret = add_extable_entry(fp, image, pass, code, ctx, dst_reg);
> + if (ret)
> + return ret;
> break;
>
> /*
>

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