[PATCH bpf v2] bpf: fix accessing bpf_sysctl.file_pos on s390

[Ilya Leoshkevich <iii@linux.ibm.com>]

"ctx:file_pos sysctl:read write ok" fails on s390 with "Read value  !=
nux". This is because verifier rewrites a complete 32-bit
bpf_sysctl.file_pos update to a partial update of the first 32 bits of
64-bit *bpf_sysctl_kern.ppos, which is not correct on big-endian
systems.

Fix by using an offset on big-endian systems.

Ditto for bpf_sysctl.file_pos reads. Currently the test does not detect
a problem there, since it expects to see 0, which it gets with high
probability in error cases, so change it to seek to offset 3 and expect
3 in bpf_sysctl.file_pos.

Fixes: e1550bfe0de4 ("bpf: Add file_pos field to bpf_sysctl ctx")
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
---
v1->v2: Merge bpf_ctx_narrow_load_shift and
bpf_ctx_narrow_access_offset.

 include/linux/filter.h                    |  8 ++++----
 kernel/bpf/cgroup.c                       | 10 ++++++++--
 kernel/bpf/verifier.c                     |  4 ++--
 tools/testing/selftests/bpf/test_sysctl.c |  9 ++++++++-
 4 files changed, 22 insertions(+), 9 deletions(-)

diff --git a/include/linux/filter.h b/include/linux/filter.h
index 92c6e31fb008..2ce57645f3cd 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -749,14 +749,14 @@ bpf_ctx_narrow_access_ok(u32 off, u32 size, u32 size_default)
 }
 
 static inline u8
-bpf_ctx_narrow_load_shift(u32 off, u32 size, u32 size_default)
+bpf_ctx_narrow_access_offset(u32 off, u32 size, u32 size_default)
 {
-	u8 load_off = off & (size_default - 1);
+	u8 access_off = off & (size_default - 1);
 
 #ifdef __LITTLE_ENDIAN
-	return load_off * 8;
+	return access_off;
 #else
-	return (size_default - (load_off + size)) * 8;
+	return size_default - (access_off + size);
 #endif
 }
 
diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
index 0a00eaca6fae..00c4647ce92a 100644
--- a/kernel/bpf/cgroup.c
+++ b/kernel/bpf/cgroup.c
@@ -1325,6 +1325,7 @@ static u32 sysctl_convert_ctx_access(enum bpf_access_type type,
 				     struct bpf_prog *prog, u32 *target_size)
 {
 	struct bpf_insn *insn = insn_buf;
+	u32 read_size;
 
 	switch (si->off) {
 	case offsetof(struct bpf_sysctl, write):
@@ -1356,7 +1357,9 @@ static u32 sysctl_convert_ctx_access(enum bpf_access_type type,
 				treg, si->dst_reg,
 				offsetof(struct bpf_sysctl_kern, ppos));
 			*insn++ = BPF_STX_MEM(
-				BPF_SIZEOF(u32), treg, si->src_reg, 0);
+				BPF_SIZEOF(u32), treg, si->src_reg,
+				bpf_ctx_narrow_access_offset(
+					0, sizeof(u32), sizeof(loff_t)));
 			*insn++ = BPF_LDX_MEM(
 				BPF_DW, treg, si->dst_reg,
 				offsetof(struct bpf_sysctl_kern, tmp_reg));
@@ -1365,8 +1368,11 @@ static u32 sysctl_convert_ctx_access(enum bpf_access_type type,
 				BPF_FIELD_SIZEOF(struct bpf_sysctl_kern, ppos),
 				si->dst_reg, si->src_reg,
 				offsetof(struct bpf_sysctl_kern, ppos));
+			read_size = bpf_size_to_bytes(BPF_SIZE(si->code));
 			*insn++ = BPF_LDX_MEM(
-				BPF_SIZE(si->code), si->dst_reg, si->dst_reg, 0);
+				BPF_SIZE(si->code), si->dst_reg, si->dst_reg,
+				bpf_ctx_narrow_access_offset(
+					0, read_size, sizeof(loff_t)));
 		}
 		*target_size = sizeof(u32);
 		break;
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index c84d83f86141..d1d4c995a9eb 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -8616,8 +8616,8 @@ static int convert_ctx_accesses(struct bpf_verifier_env *env)
 		}
 
 		if (is_narrower_load && size < target_size) {
-			u8 shift = bpf_ctx_narrow_load_shift(off, size,
-							     size_default);
+			u8 shift = bpf_ctx_narrow_access_offset(
+				off, size, size_default) * 8;
 			if (ctx_field_size <= 4) {
 				if (shift)
 					insn_buf[cnt++] = BPF_ALU32_IMM(BPF_RSH,
diff --git a/tools/testing/selftests/bpf/test_sysctl.c b/tools/testing/selftests/bpf/test_sysctl.c
index a3bebd7c68dd..abc26248a7f1 100644
--- a/tools/testing/selftests/bpf/test_sysctl.c
+++ b/tools/testing/selftests/bpf/test_sysctl.c
@@ -31,6 +31,7 @@ struct sysctl_test {
 	enum bpf_attach_type attach_type;
 	const char *sysctl;
 	int open_flags;
+	int seek;
 	const char *newval;
 	const char *oldval;
 	enum {
@@ -139,7 +140,7 @@ static struct sysctl_test tests[] = {
 			/* If (file_pos == X) */
 			BPF_LDX_MEM(BPF_W, BPF_REG_7, BPF_REG_1,
 				    offsetof(struct bpf_sysctl, file_pos)),
-			BPF_JMP_IMM(BPF_JNE, BPF_REG_7, 0, 2),
+			BPF_JMP_IMM(BPF_JNE, BPF_REG_7, 3, 2),
 
 			/* return ALLOW; */
 			BPF_MOV64_IMM(BPF_REG_0, 1),
@@ -152,6 +153,7 @@ static struct sysctl_test tests[] = {
 		.attach_type = BPF_CGROUP_SYSCTL,
 		.sysctl = "kernel/ostype",
 		.open_flags = O_RDONLY,
+		.seek = 3,
 		.result = SUCCESS,
 	},
 	{
@@ -1442,6 +1444,11 @@ static int access_sysctl(const char *sysctl_path,
 	if (fd < 0)
 		return fd;
 
+	if (test->seek && lseek(fd, test->seek, SEEK_SET) == -1) {
+		log_err("lseek(%d) failed", test->seek);
+		goto err;
+	}
+
 	if (test->open_flags == O_RDONLY) {
 		char buf[128];
 
-- 
2.21.0