[PATCH v1] kheaders: prevent `find` from seeing perl temp files

[HONG Yifan <elsk@google.com>]

Symptom:

The command

    find ... | xargs ... perl -i

occasionally triggers error messages like the following, with the build
still succeeding:

    Can't open <redacted>/kernel/.tmp_cpio_dir/include/dt-bindings/clock/XXNX4nW9: No such file or directory.

Analysis:

With strace, the root cause has been identified to be `perl -i` creating
temporary files inside $cpio_dir, which causes `find` to see the
temporary files and emit the names. `find` is likely implemented with
readdir. POSIX `readdir` says:

    If a file is removed from or added to the directory after the most
    recent call to opendir() or rewinddir(), whether a subsequent call
    to readdir() returns an entry for that file is unspecified.

So if the libc that `find` links against choose to return that entry
in readdir(), a possible sequence of events is the following:

1. find emits foo.h
2. xargs executes `perl -i foo.h`
3. perl (pid=100) creates temporary file `XXXXXXXX`
4. find sees file `XXXXXXXX` and emit it
5. PID 100 exits, cleaning up the temporary file `XXXXXXXX`
6. xargs executes `perl -i XXXXXXXX`
7. perl (pid=200) tries to read the file, but it doesn't exist any more.

... triggering the error message.

One can reproduce the bug with the following command (assuming PWD
contains the list of headers in kheaders.tar.xz)

    for i in $(seq 100); do
        find -type f -print0 |
            xargs -0 -P8 -n1 perl -pi -e 'BEGIN {undef $/;}; s/\/\*((?!SPDX).)*?\*\///smg;';
    done

With a `find` linking against musl libc, the error message is emitted
6/100 times.

The fix:

This change modifies the command so `find` only emits .h names, thereby
skipping the temporary files. Another possible fix would be to store
the results of `find` before feeding them into xargs; I didn't take this
approach because the current approach appears marginally more optimized
and involves a smaller change.

Signed-off-by: HONG Yifan <elsk@google.com>
---
 kernel/gen_kheaders.sh | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/gen_kheaders.sh b/kernel/gen_kheaders.sh
index 383fd43ac612..271e0145f406 100755
--- a/kernel/gen_kheaders.sh
+++ b/kernel/gen_kheaders.sh
@@ -84,7 +84,7 @@ for f in $dir_list;
 done | cpio --quiet -pdu $cpio_dir >/dev/null 2>&1
 
 # Remove comments except SDPX lines
-find $cpio_dir -type f -print0 |
+find $cpio_dir -type f -name '*.h' -print0 |
 	xargs -0 -P8 -n1 perl -pi -e 'BEGIN {undef $/;}; s/\/\*((?!SPDX).)*?\*\///smg;'
 
 # Create archive and try to normalize metadata for reproducibility.
-- 
2.47.0.199.ga7371fff76-goog