bpf ppc32: Add BPF_PROBE_MEM support for JIT

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>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211012123056.485795-8-hbathini@linux.ibm.com

arch/powerpc/net/bpf_jit.h

@@ -153,7 +153,11 @@ struct codegen_context {
 	unsigned int exentry_idx;
 };
 
+#ifdef CONFIG_PPC32
+#define BPF_FIXUP_LEN	3 /* Three instructions => 12 bytes */
+#else
 #define BPF_FIXUP_LEN	2 /* Two instructions => 8 bytes */
+#endif
 
 static inline void bpf_flush_icache(void *start, void *end)
 {

arch/powerpc/net/bpf_jit_comp.c

@@ -297,6 +297,8 @@ int bpf_add_extable_entry(struct bpf_prog *fp, u32 *image, int pass, struct code
 		(ctx->exentry_idx * BPF_FIXUP_LEN * 4);
 
 	fixup[0] = PPC_RAW_LI(dst_reg, 0);
+	if (IS_ENABLED(CONFIG_PPC32))
+		fixup[1] = PPC_RAW_LI(dst_reg - 1, 0); /* clear higher 32-bit register too */
 
 	fixup[BPF_FIXUP_LEN - 1] =
 		PPC_RAW_BRANCH((long)(pc + jmp_off) - (long)&fixup[BPF_FIXUP_LEN - 1]);

arch/powerpc/net/bpf_jit_comp32.c

@@ -813,9 +813,13 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, struct codegen_context *
 		 * BPF_LDX
 		 */
 		case BPF_LDX | BPF_MEM | BPF_B: /* dst = *(u8 *)(ul) (src + off) */
+		case BPF_LDX | BPF_PROBE_MEM | BPF_B:
 		case BPF_LDX | BPF_MEM | BPF_H: /* dst = *(u16 *)(ul) (src + off) */
+		case BPF_LDX | BPF_PROBE_MEM | BPF_H:
 		case BPF_LDX | BPF_MEM | BPF_W: /* dst = *(u32 *)(ul) (src + off) */
+		case BPF_LDX | BPF_PROBE_MEM | BPF_W:
 		case BPF_LDX | BPF_MEM | BPF_DW: /* dst = *(u64 *)(ul) (src + off) */
+		case BPF_LDX | BPF_PROBE_MEM | BPF_DW:
 			switch (size) {
 			case BPF_B:
 				EMIT(PPC_RAW_LBZ(dst_reg, src_reg, off));
@@ -834,6 +838,32 @@ 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;
 
 		/*