ssh signing: return an error when signature cannot be read

From: Phillip Wood <[email protected]>

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.

---
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(-)

index 947b58ad4da..d352bc286b6 100644
@@ -1043,9 +1043,11 @@ static int sign_buffer_ssh(struct strbuf *buffer, struct strbuf *signature,
 	strbuf_addbuf(&ssh_signature_filename, &buffer_file->filename);
 	strbuf_addstr(&ssh_signature_filename, ".sig");
 	if (strbuf_read_file(signature, ssh_signature_filename.buf, 0) < 0) {
-		error_errno(
+		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);
 

base-commit: a0feb8611d4c0b2b5d954efe4e98207f62223436

gitgitgadget

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 <[email protected]>

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.

---
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.

@@ 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(-)

index 947b58ad4da..b5c2bbb3b91 100644
@@ -1043,12 +1043,11 @@ static int sign_buffer_ssh(struct strbuf *buffer, struct strbuf *signature,
 	strbuf_addbuf(&ssh_signature_filename, &buffer_file->filename);
 	strbuf_addstr(&ssh_signature_filename, ".sig");
 	if (strbuf_read_file(signature, ssh_signature_filename.buf, 0) < 0) {
-		error_errno(
+		ret = error_errno(
 			_("failed reading ssh signing data buffer from '%s'"),
 			ssh_signature_filename.buf);
+		goto out;
 	}
-	unlink_or_warn(ssh_signature_filename.buf);
-
 	/* Strip CR from the line endings, in case we are on Windows. */
 	remove_cr_after(signature, bottom);
 

@@ -1057,6 +1056,8 @@ 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);

base-commit: a0feb8611d4c0b2b5d954efe4e98207f62223436

gitgitgadget

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

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

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