"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@xxxxxxxxxxxxx>
---
include/linux/filter.h | 10 ++++++++++
kernel/bpf/cgroup.c | 9 +++++++--
tools/testing/selftests/bpf/test_sysctl.c | 9 ++++++++-
3 files changed, 25 insertions(+), 3 deletions(-)
diff --git a/include/linux/filter.h b/include/linux/filter.h
index 92c6e31fb008..94e81c56d81c 100644
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -760,6 +760,16 @@ bpf_ctx_narrow_load_shift(u32 off, u32 size, u32 size_default)
#endif
}
+static inline s16
+bpf_ctx_narrow_access_offset(size_t variable_size, size_t access_size)
+{
+#ifdef __LITTLE_ENDIAN
+ return 0;
+#else
+ return variable_size - access_size;
+#endif
+}
+
#define bpf_ctx_wide_access_ok(off, size, type, field) \
(size == sizeof(__u64) && \
off >= offsetof(type, field) && \
diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
index 0a00eaca6fae..b835fbb13ea8 100644
--- a/kernel/bpf/cgroup.c
+++ b/kernel/bpf/cgroup.c
@@ -1356,7 +1356,9 @@ 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(
+ sizeof(loff_t), sizeof(u32)));
*insn++ = BPF_LDX_MEM(
BPF_DW, treg, si->dst_reg,
offsetof(struct bpf_sysctl_kern, tmp_reg));
@@ -1366,7 +1368,10 @@ static u32 sysctl_convert_ctx_access(enum bpf_access_type type,
si->dst_reg, si->src_reg,
offsetof(struct bpf_sysctl_kern, ppos));
*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(
+ sizeof(loff_t),
+ bpf_size_to_bytes(BPF_SIZE(si->code))));
}
*target_size = sizeof(u32);
break;
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
