linux-kernel - Re: [PATCH RFC] gen_init_cpio: Do fsync() only on regular files

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Message-ID: <20251007121719.45090b21.ddiss@suse.de>
Date: Tue, 7 Oct 2025 12:17:19 +1100
From: David Disseldorp <ddiss@...e.de>
To: Dmitry Safonov via B4 Relay <devnull+dima.arista.com@...nel.org>
Cc: dima@...sta.com, Nathan Chancellor <nathan@...nel.org>, Nicolas Schier
 <nicolas.schier@...ux.dev>, linux-kbuild@...r.kernel.org,
 linux-kernel@...r.kernel.org, Nicolas Schier <nsc@...nel.org>
Subject: Re: [PATCH RFC] gen_init_cpio: Do fsync() only on regular files

Thanks for reporting this regression, Dmitry...

On Tue, 07 Oct 2025 00:55:03 +0100, Dmitry Safonov via B4 Relay wrote:

> From: Dmitry Safonov <dima@...sta.com>
> 
> Here at Arista gen_init_cpio is used in testing in order to create
> an initramfs for specific tests. Most notably, there is a test that does
> essentially a fork-bomb in kdump/panic kernel, replacing build-time
> generated init script: instead of doing makedumpfile, it does call
> shell tests.
> 
> In comparison to usr/Makefile, which creates an intermediate .cpio file,
> the Makefile that generates initrd for tests is a one-liner:
> > file lib/kdump ${src_dir}/oom-crashkernel 0644 0 0 | usr/gen_init_cpio /dev/stdin | xz -z -c -e -Ccrc32 > ${target_dir}/oom-crashkernel.initrd  
> 
> As fsync() on a pipe fd returns -EINVAL, that stopped working.
> Check that outfd is a regular file descriptor before calling fsync().
> 
> Sending this as RFC as these are local tests, rather than upstream ones,
> unfortunately. Yet, the fix is trivial and increases correctness of
> gen_init_cpio (and maybe saves some time for another person debugging
> it). A workaround to use temporary cpio file is also trivial, so not
> insisting on merging.

The code change looks fine, but the commit message is a bit verbose IMO.
Please drop the first and last paragraphs. The reproducer could also be
simplified to e.g.
echo | usr/gen_init_cpio /dev/stdin > /dev/null

With that:
Reviewed-by: David Disseldorp <ddiss@...e.de>

> Fixes: ae18b94099b0 ("gen_init_cpio: support -o <output_file> parameter")
> Cc: David Disseldorp <ddiss@...e.de>
> Cc: Nicolas Schier <nsc@...nel.org>
> Signed-off-by: Dmitry Safonov <dima@...sta.com>
> ---
>  usr/gen_init_cpio.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/usr/gen_init_cpio.c b/usr/gen_init_cpio.c
> index 75e9561ba31392e12536e76a918245a8ea07f9b8..845e2d92f0e56b02ba5fc12ecd243ce99c53f552 100644
> --- a/usr/gen_init_cpio.c
> +++ b/usr/gen_init_cpio.c
> @@ -6,6 +6,7 @@
>  #include <stdbool.h>
>  #include <sys/types.h>
>  #include <sys/stat.h>
> +#include <sys/socket.h>
>  #include <string.h>
>  #include <unistd.h>
>  #include <time.h>
> @@ -112,6 +113,9 @@ static int cpio_trailer(void)
>  	    push_pad(padlen(offset, 512)) < 0)
>  		return -1;
>  
> +	if (!isfdtype(outfd, S_IFREG))
> +		return 0;
> +
>  	return fsync(outfd);
>  }

Another option would be to catch the EINVAL error, e.g.

--- a/usr/gen_init_cpio.c
+++ b/usr/gen_init_cpio.c
@@ -112,7 +112,10 @@ static int cpio_trailer(void)
            push_pad(padlen(offset, 512)) < 0)
                return -1;
 
-       return fsync(outfd);
+       if (fsync(outfd) < 0 && errno != EINVAL)
+               return -1;
+
+       return 0;
 }

It may be a little portable than isfdtype(), but I don't feel strongly
either way.

Thanks, David