patchssh signing: return an error when signature cannot be read

From: Phillip Wood <phillip.wood@dunelm.org.uk>

If the signature file cannot be read we print an error message but do not return an error to the caller. In practice it seems unlikely that the file would be unreadable if the call to ssh-keygen succeeds. If we cannot read the file it may be missing so ignore any errors from unlink() when we try to remove it.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
---
    ssh signing: return an error when signature cannot be read
    
    This patch is based on maint. In the longer term the code could be
    simplified by using pipes rather than tempfiles as we do for gpg.
    ssh-keygen has supported reading the data to be signed from stdin and
    writing the signature to stdout since it introduced signing.

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1371%2Fphillipwood%2Fssh-signing-return-error-on-missing-signature-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1371/phillipwood/ssh-signing-return-error-on-missing-signature-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/1371

 gpg-interface.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

"Phillip Wood via GitGitGadget" <gitgitgadget@gmail.com> writes:

OK. Not removing may help diagnose what the problem is, but going that route needs to add code to report what file is deliberately left for inspection. I do not know how valuable that would be to help human debuggers --- the user presumably have the original material (e.g. a signed tag object) anyway, and the human debugger probably needs to have access to both the original material and what is fed to the gpg-interface API. If they are chasing a reproducible bug, the latter should be recreatable by the human debugger from the former, so removing would not hurt the debuggability that much.

On the other hand, we can still report if the reason we cannot remove is not ENOENT, though.

Thanks.

From: Phillip Wood <phillip.wood@dunelm.org.uk>

If the signature file cannot be read we print an error message but do not return an error to the caller. In practice it seems unlikely that the file would be unreadable if the call to ssh-keygen succeeds.

The unlink_or_warn() call is moved to the end of the function so that we always try and remove the signature file. This isn't strictly necessary at the moment but it protects us against any extra code being added between trying to read the signature file and the cleanup at the end of the function in the future. unlink_or_warn() only prints a warning if it exists and cannot be removed.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
---
    ssh signing: return an error when signature cannot be read
    
    Thanks to Junio for his comments. I've updated the patch to always use
    unlink_or_warn() to remove the signature file as it does not warn on
    missing files.
    
    V1 cover letter
    
    This patch is based on maint. In the longer term the code could be
    simplified by using pipes rather than tempfiles as we do for gpg.
    ssh-keygen has supported reading the data to be signed from stdin and
    writing the signature to stdout since it introduced signing.

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1371%2Fphillipwood%2Fssh-signing-return-error-on-missing-signature-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1371/phillipwood/ssh-signing-return-error-on-missing-signature-v2
Pull-Request: https://github.com/gitgitgadget/git/pull/1371

Range-diff vs v1:

 1:  6f569ac0f48 ! 1:  1db8af68fce ssh signing: return an error when signature cannot be read
     @@ Commit message
      
          If the signature file cannot be read we print an error message but do
          not return an error to the caller. In practice it seems unlikely that
     -    the file would be unreadable if the call to ssh-keygen succeeds. If we
     -    cannot read the file it may be missing so ignore any errors from
     -    unlink() when we try to remove it.
     +    the file would be unreadable if the call to ssh-keygen succeeds.
     +
     +    The unlink_or_warn() call is moved to the end of the function so that
     +    we always try and remove the signature file. This isn't strictly
     +    necessary at the moment but it protects us against any extra code
     +    being added between trying to read the signature file and the cleanup
     +    at the end of the function in the future. unlink_or_warn() only prints
     +    a warning if it exists and cannot be removed.
      
          Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
      
     @@ gpg-interface.c: static int sign_buffer_ssh(struct strbuf *buffer, struct strbuf
      +		ret = error_errno(
       			_("failed reading ssh signing data buffer from '%s'"),
       			ssh_signature_filename.buf);
     -+		unlink(ssh_signature_filename.buf);
      +		goto out;
       	}
     - 	unlink_or_warn(ssh_signature_filename.buf);
     +-	unlink_or_warn(ssh_signature_filename.buf);
     +-
     + 	/* Strip CR from the line endings, in case we are on Windows. */
     + 	remove_cr_after(signature, bottom);
       
     +@@ gpg-interface.c: out:
     + 		delete_tempfile(&key_file);
     + 	if (buffer_file)
     + 		delete_tempfile(&buffer_file);
     ++	if (ssh_signature_filename.len)
     ++		unlink_or_warn(ssh_signature_filename.buf);
     + 	strbuf_release(&signer_stderr);
     + 	strbuf_release(&ssh_signature_filename);
     + 	FREE_AND_NULL(ssh_signing_key_file);

 gpg-interface.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

On 04.10.2022 10:01, Phillip Wood via GitGitGadget wrote:

Sounds sensible and the change looks good to me.

The ssh-keygen call is already using stdin for the content to sign or verify. The signature and the signing key need to be files passed as parameters to ssh-keygen. I'm not aware of any other option of providing them to it.

Cheers, Fabian

Hi Fabian

On 06/10/2022 09:28, Fabian Stelzer wrote:

We use stdin for the content when verifying but not when signing

	strvec_pushl(&signer.args, use_format->program,
		     "-Y", "sign",
		     "-n", "git",
		     "-f", ssh_signing_key_file,
		     buffer_file->filename.buf,
		     NULL);

	sigchain_push(SIGPIPE, SIG_IGN);
	ret = pipe_command(&signer, NULL, 0, NULL, 0, &signer_stderr, 0);
	sigchain_pop(SIGPIPE);

Note that when verifying with -Y check-novalidate there is a missing call to sigchain_push(SIGPIPE, SIG_IGN) as we are passing data over stdin so need to ignore SIGPIPE.

Best Wishes

Phillip

On 06.10.2022 14:05, Phillip Wood wrote:

Hm, true. I was kinda sure it both used stdin/out. I'm short on time at the moment and can't really work on git stuff. But I hope i can at the end of the year. There's a few more todos on my list.

Cheers, Fabian