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bpf: Fix issue with bpf preload module taking over stdout/stdin of kernel.

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In cb80ddc671 ("bpf: Convert bpf_preload.ko to use light skeleton.")
BPF preload was switched from user mode process to use in-kernel light
skeleton instead. However, in the kernel context, early in the boot
sequence, the first available FD can start from 0, instead of normally
3 for user mode process. So FDs 0 and 1 are then used for loaded BPF
programs and prevent init process from setting up stdin/stdout/stderr on
FD 0, 1, and 2 as expected.

Before the fix:

ls -lah /proc/1/fd/*

lrwx------1 root root 64 Feb 23 17:20 /proc/1/fd/0 -> /dev/null
lrwx------ 1 root root 64 Feb 23 17:20 /proc/1/fd/1 -> /dev/null
lrwx------ 1 root root 64 Feb 23 17:20 /proc/1/fd/2 -> /dev/console
lrwx------ 1 root root 64 Feb 23 17:20 /proc/1/fd/6 -> /dev/console
lrwx------ 1 root root 64 Feb 23 17:20 /proc/1/fd/7 -> /dev/console

After the fix:

ls -lah /proc/1/fd/*

lrwx------ 1 root root 64 Feb 24 21:23 /proc/1/fd/0 -> /dev/console
lrwx------ 1 root root 64 Feb 24 21:23 /proc/1/fd/1 -> /dev/console
lrwx------ 1 root root 64 Feb 24 21:23 /proc/1/fd/2 -> /dev/console

Fix by closing prog FDs after initialization. struct bpf_prog's
themselves are kept alive through direct kernel references taken with
bpf_link_get_from_fd().

Fixes: cb80ddc671 ("bpf: Convert bpf_preload.ko to use light skeleton.")
Signed-off-by: Yucong Sun <fallentree@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220225185923.2535519-1-fallentree@fb.com
This commit is contained in:
Yucong Sun 2022-02-25 10:59:24 -08:00 committed by Andrii Nakryiko
parent 08d4dba6ae
commit 80bebebdac
1 changed files with 7 additions and 0 deletions
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7
kernel/bpf/preload/bpf_preload_kern.c
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@ -54,6 +54,13 @@ static int load_skel(void)
err = PTR_ERR(progs_link);
goto out;
}
/* Avoid taking over stdin/stdout/stderr of init process. Zeroing out
* makes skel_closenz() a no-op later in iterators_bpf__destroy().
*/
close_fd(skel->links.dump_bpf_map_fd);
skel->links.dump_bpf_map_fd = 0;
close_fd(skel->links.dump_bpf_prog_fd);
skel->links.dump_bpf_prog_fd = 0;
return 0;
out:
free_links_and_skel();