Running the eBPF test_verifier leads to random errors looking like this:

[ 6525.735488] Unexpected kernel BRK exception at EL1
[ 6525.735502] Internal error: ptrace BRK handler: f2000100 [#1] SMP
[ 6525.741609] Modules linked in: nls_utf8 cifs libdes libarc4 dns_resolver fscache binfmt_misc nls_ascii nls_cp437 vfat fat aes_ce_blk crypto_simd cryptd aes_ce_cipher ghash_ce gf128mul efi_pstore sha2_ce sha256_arm64 sha1_ce evdev efivars efivarfs ip_tables x_tables autofs4 btrfs blake2b_generic xor xor_neon zstd_compress raid6_pq libcrc32c crc32c_generic ahci xhci_pci libahci xhci_hcd igb libata i2c_algo_bit nvme realtek usbcore nvme_core scsi_mod t10_pi netsec mdio_devres of_mdio gpio_keys fixed_phy libphy gpio_mb86s7x
[ 6525.787760] CPU: 3 PID: 7881 Comm: test_verifier Tainted: G        W         5.9.0-rc1+ #47
[ 6525.796111] Hardware name: Socionext SynQuacer E-series DeveloperBox, BIOS build #1 Jun  6 2020
[ 6525.804812] pstate: 20000005 (nzCv daif -PAN -UAO BTYPE=--)
[ 6525.810390] pc : bpf_prog_c3d01833289b6311_F+0xc8/0x9f4
[ 6525.815613] lr : bpf_prog_d53bb52e3f4483f9_F+0x38/0xc8c
[ 6525.820832] sp : ffff8000130cbb80
[ 6525.824141] x29: ffff8000130cbbb0 x28: 0000000000000000
[ 6525.829451] x27: 000005ef6fcbf39b x26: 0000000000000000
[ 6525.834759] x25: ffff8000130cbb80 x24: ffff800011dc7038
[ 6525.840067] x23: ffff8000130cbd00 x22: ffff0008f624d080
[ 6525.845375] x21: 0000000000000001 x20: ffff800011dc7000
[ 6525.850682] x19: 0000000000000000 x18: 0000000000000000
[ 6525.855990] x17: 0000000000000000 x16: 0000000000000000
[ 6525.861298] x15: 0000000000000000 x14: 0000000000000000
[ 6525.866606] x13: 0000000000000000 x12: 0000000000000000
[ 6525.871913] x11: 0000000000000001 x10: ffff8000000a660c
[ 6525.877220] x9 : ffff800010951810 x8 : ffff8000130cbc38
[ 6525.882528] x7 : 0000000000000000 x6 : 0000009864cfa881
[ 6525.887836] x5 : 00ffffffffffffff x4 : 002880ba1a0b3e9f
[ 6525.893144] x3 : 0000000000000018 x2 : ffff8000000a4374
[ 6525.898452] x1 : 000000000000000a x0 : 0000000000000009
[ 6525.903760] Call trace:
[ 6525.906202]  bpf_prog_c3d01833289b6311_F+0xc8/0x9f4
[ 6525.911076]  bpf_prog_d53bb52e3f4483f9_F+0x38/0xc8c
[ 6525.915957]  bpf_dispatcher_xdp_func+0x14/0x20
[ 6525.920398]  bpf_test_run+0x70/0x1b0
[ 6525.923969]  bpf_prog_test_run_xdp+0xec/0x190
[ 6525.928326]  __do_sys_bpf+0xc88/0x1b28
[ 6525.932072]  __arm64_sys_bpf+0x24/0x30
[ 6525.935820]  el0_svc_common.constprop.0+0x70/0x168
[ 6525.940607]  do_el0_svc+0x28/0x88
[ 6525.943920]  el0_sync_handler+0x88/0x190
[ 6525.947838]  el0_sync+0x140/0x180
[ 6525.951154] Code: d4202000 d4202000 d4202000 d4202000 (d4202000)
[ 6525.957249] ---[ end trace cecc3f93b14927e2 ]---

The reason is the offset[] creation and later usage while building
the eBPF body. The code currently omits the first instruction, since
build_insn() will increase our ctx->idx before saving it.
That was fine up until bounded eBPF loops were introduced. After that
introduction, offset[0] must be the offset of the end of prologue which
is the start of the 1st insn while, offset[n] holds the
offset of the end of n-th insn.

When "taken loop with back jump to 1st insn" test runs, it will
eventually call bpf2a64_offset(-1, 2, ctx). Since negative indexing is
permitted, the current outcome depends on the value stored in
ctx->offset[-1], which has nothing to do with our array.
If the value happens to be 0 the tests will work. If not this error
triggers.

7c2e988 ("bpf: fix x64 JIT code generation for jmp to 1st insn")
fixed an indentical bug on x86 when eBPF bounded loops were introduced.

So let's fix it by creating the ctx->offset[] correctly in the first
place and account for the first instruction while calculating the arm
instruction offsets.

Fixes: 2589726 ("bpf: introduce bounded loops")
Reported-by: Naresh Kamboju <[email protected]>
Reported-by: Jiri Olsa <[email protected]>
Co-developed-by: Jean-Philippe Brucker <[email protected]>
Signed-off-by: Jean-Philippe Brucker <[email protected]>
Co-developed-by: Yauheni Kaliuta <[email protected]>
Signed-off-by: Yauheni Kaliuta <[email protected]>
Signed-off-by: Ilias Apalodimas <[email protected]>
---
Changes since v1:
 - Added Co-developed-by, Reported-by and Fixes tags correctly
 - Describe the expected context of ctx->offset[] in comments

 arch/arm64/net/bpf_jit_comp.c | 28 ++++++++++++++++++++--------
 1 file changed, 20 insertions(+), 8 deletions(-)

Author: apalos

arch/arm64/net/bpf_jit_comp.c

@@ -143,9 +143,13 @@ static inline void emit_addr_mov_i64(const int reg, const u64 val,
 	}
 }
 
-static inline int bpf2a64_offset(int bpf_to, int bpf_from,
+static inline int bpf2a64_offset(int bpf_insn, int off,
 				 const struct jit_ctx *ctx)
 {
+	/* arm64 offset is relative to the branch instruction */
+	int bpf_from = bpf_insn + 1;
+	/* BPF JMP offset is relative to the next instruction */
+	int bpf_to = bpf_insn + off + 1;
 	int to = ctx->offset[bpf_to];
 	/* -1 to account for the Branch instruction */
 	int from = ctx->offset[bpf_from] - 1;
@@ -642,7 +646,7 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx,
 
 	/* JUMP off */
 	case BPF_JMP | BPF_JA:
-		jmp_offset = bpf2a64_offset(i + off, i, ctx);
+		jmp_offset = bpf2a64_offset(i, off, ctx);
 		check_imm26(jmp_offset);
 		emit(A64_B(jmp_offset), ctx);
 		break;
@@ -669,7 +673,7 @@ static int build_insn(const struct bpf_insn *insn, struct jit_ctx *ctx,
 	case BPF_JMP32 | BPF_JSLE | BPF_X:
 		emit(A64_CMP(is64, dst, src), ctx);
 emit_cond_jmp:
-		jmp_offset = bpf2a64_offset(i + off, i, ctx);
+		jmp_offset = bpf2a64_offset(i, off, ctx);
 		check_imm19(jmp_offset);
 		switch (BPF_OP(code)) {
 		case BPF_JEQ:
@@ -912,18 +916,26 @@ static int build_body(struct jit_ctx *ctx, bool extra_pass)
 		const struct bpf_insn *insn = &prog->insnsi[i];
 		int ret;
 
+		/*
+		 * offset[0] offset of the end of prologue, start of the
+		 * first insn.
+		 * offset[x] - offset of the end of x insn.
+		 */
+		if (ctx->image == NULL)
+			ctx->offset[i] = ctx->idx;
+
 		ret = build_insn(insn, ctx, extra_pass);
 		if (ret > 0) {
 			i++;
 			if (ctx->image == NULL)
-				ctx->offset[i] = ctx->idx;
+				ctx->offset[i] = ctx->offset[i - 1];
 			continue;
 		}
-		if (ctx->image == NULL)
-			ctx->offset[i] = ctx->idx;
 		if (ret)
 			return ret;
 	}
+	if (ctx->image == NULL)
+		ctx->offset[i] = ctx->idx;
 
 	return 0;
 }
@@ -1002,7 +1014,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
 	memset(&ctx, 0, sizeof(ctx));
 	ctx.prog = prog;
 
-	ctx.offset = kcalloc(prog->len, sizeof(int), GFP_KERNEL);
+	ctx.offset = kcalloc(prog->len + 1, sizeof(int), GFP_KERNEL);
 	if (ctx.offset == NULL) {
 		prog = orig_prog;
 		goto out_off;
@@ -1089,7 +1101,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
 	prog->jited_len = prog_size;
 
 	if (!prog->is_func || extra_pass) {
-		bpf_prog_fill_jited_linfo(prog, ctx.offset);
+		bpf_prog_fill_jited_linfo(prog, ctx.offset + 1);
 out_off:
 		kfree(ctx.offset);
 		kfree(jit_data);