bpf: Implement bpf_seq_read() for bpf iterator
bpf iterator uses seq_file to provide a lossless way to transfer data to user space. But we want to call bpf program after all objects have been traversed, and bpf program may write additional data to the seq_file buffer. The current seq_read() does not work for this use case. Besides allowing stop() function to write to the buffer, the bpf_seq_read() also fixed the buffer size to one page. If any single call of show() or stop() will emit data more than one page to cause overflow, -E2BIG error code will be returned to user space. Signed-off-by: NYonghong Song <yhs@fb.com> Signed-off-by: NAlexei Starovoitov <ast@kernel.org> Acked-by: NAndrii Nakryiko <andriin@fb.com> Link: https://lore.kernel.org/bpf/20200509175904.2475468-1-yhs@fb.com
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