bpf: bpf_probe_read_kernel_str() has to return amount of data read on success

During recent refactorings, bpf_probe_read_kernel_str() started returning 0 on
success, instead of amount of data successfully read. This majorly breaks
applications relying on bpf_probe_read_kernel_str() and bpf_probe_read_str()
and their results. Fix this by returning actual number of bytes read.

Fixes: 8d92db5c04d1 ("bpf: rework the compat kernel probe handling")
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20200616050432.1902042-1-andriin@fb.com

diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index e729c9e..a3ac7de 100644 (file)
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -241,7 +241,7 @@ bpf_probe_read_kernel_str_common(void *dst, u32 size, const void *unsafe_ptr)
        if (unlikely(ret < 0))
                goto fail;
 
-       return 0;
+       return ret;
 fail:
        memset(dst, 0, size);
        return ret;