bpf: fix accessing bpf_sysctl.file_pos on s390

"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>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20190816105300.49035-1-iii@linux.ibm.com/

1 files changed, 8 insertions, 1 deletions

diff --git a/tools/testing/selftests/bpf/test_sysctl.c b/tools/testing/selftests/bpf/test_sysctl.c
index fc33ae36b760..4f8ec1f10a80 100644
--- a/tools/testing/selftests/bpf/test_sysctl.c
+++ b/tools/testing/selftests/bpf/test_sysctl.c
@@ -32,6 +32,7 @@ struct sysctl_test {
         enum bpf_attach_type attach_type;
         const char *sysctl;
         int open_flags;
+        int seek;
         const char *newval;
         const char *oldval;
         enum {
@@ -140,7 +141,7 @@ 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),
@@ -153,6 +154,7 @@ static struct sysctl_test tests[] = {
                 .attach_type = BPF_CGROUP_SYSCTL,
                 .sysctl = "kernel/ostype",
                 .open_flags = O_RDONLY,
+                .seek = 3,
                 .result = SUCCESS,
         },
         {
@@ -1481,6 +1483,11 @@ 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];