[PATCH v2 7/8] bpf ppc32: Add BPF_PROBE_MEM support for JIT

[Hari Bathini <hbathini@linux.ibm.com>]

BPF load instruction with BPF_PROBE_MEM mode can cause a fault
inside kernel. Append exception table for such instructions
within BPF program.

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 within BPF program. fixup section contains 3 instructions,
first 2 instructions clear dest_reg (lower & higher 32-bit registers)
and last instruction 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 -->| lwz   r28,4(r4)  |
             |                  |
             |                  |
   0x40ac -->| lwz  r3,0(r24)   |
             | lwz  r4,4(r24)   |
             |                  |
             |                  |
             |------------------|
   0x4278 -->| li  r28,0        |  \
             | li  r27,0        |  | fixup entry
             | b   0x4024       |  /
   0x4284 -->| li  r4,0         |
             | li  r3,0         |
             | b   0x40b4       |
             |------------------|
   0x4290 -->| insn=0xfffffd90  |  \ extable entry
             | fixup=0xffffffe4 |  /
   0x4298 -->| insn=0xfffffe14  |
             | fixup=0xffffffe8 |
             +------------------+

   (Addresses shown here are chosen random, not real)

Signed-off-by: Hari Bathini <hbathini@linux.ibm.com>
---

Changes in v2:
* New patch to add BPF_PROBE_MEM support for PPC32.


 arch/powerpc/net/bpf_jit.h        |  7 +++++
 arch/powerpc/net/bpf_jit_comp.c   | 50 +++++++++++++++++++++++++++++++
 arch/powerpc/net/bpf_jit_comp32.c | 30 +++++++++++++++++++
 arch/powerpc/net/bpf_jit_comp64.c | 48 ++---------------------------
 4 files changed, 89 insertions(+), 46 deletions(-)

diff --git a/arch/powerpc/net/bpf_jit.h b/arch/powerpc/net/bpf_jit.h
index 6357c71c26eb..6a591ef88006 100644
--- a/arch/powerpc/net/bpf_jit.h
+++ b/arch/powerpc/net/bpf_jit.h
@@ -144,7 +144,11 @@ struct codegen_context {
 	unsigned int exentry_idx;
 };
 
+#ifdef CONFIG_PPC32
+#define BPF_FIXUP_LEN	12 /* Three instructions */
+#else
 #define BPF_FIXUP_LEN	8 /* Two instructions */
+#endif
 
 static inline void bpf_flush_icache(void *start, void *end)
 {
@@ -174,6 +178,9 @@ 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);
 
+int bpf_add_extable_entry(struct bpf_prog *fp, u32 *image, int pass, struct codegen_context *ctx,
+			  int insn_idx, int jmp_off, int dst_reg);
+
 #endif
 
 #endif
diff --git a/arch/powerpc/net/bpf_jit_comp.c b/arch/powerpc/net/bpf_jit_comp.c
index e92bd79d3bac..a1753b8c78c8 100644
--- a/arch/powerpc/net/bpf_jit_comp.c
+++ b/arch/powerpc/net/bpf_jit_comp.c
@@ -271,3 +271,53 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *fp)
 
 	return fp;
 }
+
+/*
+ * The caller should check for (BPF_MODE(code) == BPF_PROBE_MEM) before calling
+ * this function, as this only applies to BPF_PROBE_MEM, for now.
+ */
+int bpf_add_extable_entry(struct bpf_prog *fp, u32 *image, int pass, struct codegen_context *ctx,
+			  int insn_idx, int jmp_off, 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)
+		return 0;
+
+	if (!fp->aux->extable ||
+	    WARN_ON_ONCE(ctx->exentry_idx >= fp->aux->num_exentries))
+		return -EINVAL;
+
+	pc = (unsigned long)&image[insn_idx];
+
+	fixup = (void *)fp->aux->extable -
+		(fp->aux->num_exentries * BPF_FIXUP_LEN) +
+		(ctx->exentry_idx * BPF_FIXUP_LEN);
+
+	fixup[0] = PPC_RAW_LI(dst_reg, 0);
+#ifdef CONFIG_PPC32
+	fixup[1] = PPC_RAW_LI(dst_reg - 1, 0); /* clear higher 32-bit register too */
+	fixup[2] = PPC_RAW_BRANCH((long)(pc + jmp_off) - (long)&fixup[2]);
+#else
+	fixup[1] = PPC_RAW_BRANCH((long)(pc + jmp_off) - (long)&fixup[1]);
+#endif
+
+	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;
+}
diff --git a/arch/powerpc/net/bpf_jit_comp32.c b/arch/powerpc/net/bpf_jit_comp32.c
index 94641b7be387..c6262289dcc4 100644
--- a/arch/powerpc/net/bpf_jit_comp32.c
+++ b/arch/powerpc/net/bpf_jit_comp32.c
@@ -811,12 +811,16 @@ 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:
 		/* dst = *(u16 *)(ul) (src + off) */
 		case BPF_LDX | BPF_MEM | BPF_H:
+		case BPF_LDX | BPF_PROBE_MEM | BPF_H:
 		/* dst = *(u32 *)(ul) (src + off) */
 		case BPF_LDX | BPF_MEM | BPF_W:
+		case BPF_LDX | BPF_PROBE_MEM | BPF_W:
 		/* dst = *(u64 *)(ul) (src + off) */
 		case BPF_LDX | BPF_MEM | BPF_DW:
+		case BPF_LDX | BPF_PROBE_MEM | BPF_DW:
 			switch (size) {
 			case BPF_B:
 				EMIT(PPC_RAW_LBZ(dst_reg, src_reg, off));
@@ -835,6 +839,32 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, struct codegen_context *
 
 			if ((size != BPF_DW) && !fp->aux->verifier_zext)
 				EMIT(PPC_RAW_LI(dst_reg_h, 0));
+
+			if (BPF_MODE(code) == BPF_PROBE_MEM) {
+				int insn_idx = ctx->idx - 1;
+				int jmp_off = 4;
+
+				/*
+				 * In case of BPF_DW, two lwz instructions are emitted, one
+				 * for higher 32-bit and another for lower 32-bit. So, set
+				 * ex->insn to the first of the two and jump over both
+				 * instructions in fixup.
+				 *
+				 * Similarly, with !verifier_zext, two instructions are
+				 * emitted for BPF_B/H/W case. So, set ex-insn to the
+				 * instruction that could fault and skip over both
+				 * instructions.
+				 */
+				if ((size == BPF_DW) || !fp->aux->verifier_zext) {
+					insn_idx -= 1;
+					jmp_off += 4;
+				}
+
+				ret = bpf_add_extable_entry(fp, image, pass, ctx, insn_idx,
+							    jmp_off, dst_reg);
+				if (ret)
+					return ret;
+			}
 			break;
 
 		/*
diff --git a/arch/powerpc/net/bpf_jit_comp64.c b/arch/powerpc/net/bpf_jit_comp64.c
index eb28dbc67151..10cc9f04843c 100644
--- a/arch/powerpc/net/bpf_jit_comp64.c
+++ b/arch/powerpc/net/bpf_jit_comp64.c
@@ -270,51 +270,6 @@ static void bpf_jit_emit_tail_call(u32 *image, struct codegen_context *ctx, u32
 	/* out: */
 }
 
-/*
- * The caller should check for (BPF_MODE(code) == BPF_PROBE_MEM) before calling
- * this function, as this only applies to BPF_PROBE_MEM, for now.
- */
-static int bpf_add_extable_entry(struct bpf_prog *fp, u32 *image, int pass,
-				 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)
-		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];
-
-	fixup = (void *)fp->aux->extable -
-		(fp->aux->num_exentries * BPF_FIXUP_LEN) +
-		(ctx->exentry_idx * BPF_FIXUP_LEN);
-
-	fixup[0] = PPC_RAW_LI(dst_reg, 0);
-	fixup[1] = 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, int pass)
@@ -811,7 +766,8 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, struct codegen_context *
 				addrs[++i] = ctx->idx * 4;
 
 			if (BPF_MODE(code) == BPF_PROBE_MEM) {
-				ret = bpf_add_extable_entry(fp, image, pass, ctx, dst_reg);
+				ret = bpf_add_extable_entry(fp, image, pass, ctx, ctx->idx - 1,
+							    4, dst_reg);
 				if (ret)
 					return ret;
 			}
-- 
2.31.1