[PATCH 0/3] Correct credential helper discrepancies handling input

[PATCH 0/3] Correct credential helper discrepancies handling input

[Matthew John Cheetham via GitGitGadget]

Whilst working in the credential helper and auth space I happened to notice
that the behaviour of some of the credential helpers in contrib/credential/
is not consistent.

Specifically both the git-credential-wincred and git-credential-netrc
helpers die when they receive unknown credential property keys, which is
contrary to the behaviour of all the other in-tree helpers including:
git-credential-cache, -store, -libsecret, -gnome-keyring, -osxkeychain.

Also update the git-credential-osxkeychain helper to include a comment
making it's behaviour explicit in ignoring unknown keys, as per other
helpers.

Matthew John Cheetham (3):
  wincred: ignore unknown lines (do not die)
  netrc: ignore unknown lines (do not die)
  osxkeychain: clarify that we ignore unknown lines

 contrib/credential/netrc/git-credential-netrc.perl         | 5 ++++-
 .../credential/osxkeychain/git-credential-osxkeychain.c    | 5 +++++
 contrib/credential/wincred/git-credential-wincred.c        | 7 +++++--
 3 files changed, 14 insertions(+), 3 deletions(-)


base-commit: dd3f6c4cae7e3b15ce984dce8593ff7569650e24
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1363%2Fmjcheetham%2Fcredhelper-fixes-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1363/mjcheetham/credhelper-fixes-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/1363
-- 
gitgitgadget

[PATCH 1/3] wincred: ignore unknown lines (do not die)

[Matthew John Cheetham via GitGitGadget]

From: Matthew John Cheetham <mjcheetham@outlook.com>

It is the expectation that credential helpers be liberal in what they
accept and conservative in what they return, to allow for future growth
and evolution of the protocol/interaction.

All of the other helpers (store, cache, osxkeychain, libsecret,
gnome-keyring) except `netrc` currently ignore any credential lines
that are not recognised, whereas the Windows helper (wincred) instead
dies.

Fix the discrepancy and ignore unknown lines in the wincred helper.

Signed-off-by: Matthew John Cheetham <mjcheetham@outlook.com>
---
 contrib/credential/wincred/git-credential-wincred.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/contrib/credential/wincred/git-credential-wincred.c b/contrib/credential/wincred/git-credential-wincred.c
index 5091048f9c6..ead6e267c78 100644
--- a/contrib/credential/wincred/git-credential-wincred.c
+++ b/contrib/credential/wincred/git-credential-wincred.c
@@ -278,8 +278,11 @@ static void read_credential(void)
 			wusername = utf8_to_utf16_dup(v);
 		} else if (!strcmp(buf, "password"))
 			password = utf8_to_utf16_dup(v);
-		else
-			die("unrecognized input");
+		/*
+		 * Ignore other lines; we don't know what they mean, but
+		 * this future-proofs us when later versions of git do
+		 * learn new lines, and the helpers are updated to match.
+		 */
 	}
 }
 
-- 
gitgitgadget

Re: [PATCH 1/3] wincred: ignore unknown lines (do not die)

[Junio C Hamano]

"Matthew John Cheetham via GitGitGadget" <gitgitgadget@gmail.com>
writes:

> From: Matthew John Cheetham <mjcheetham@outlook.com>
>
> It is the expectation that credential helpers be liberal in what they
> accept and conservative in what they return, to allow for future growth
> and evolution of the protocol/interaction.

That is nice in principle, and the updated code below may work well
with existing "other side of the connection" (codepaths in "git"
that asks credential API to talk to the helpers), but I am not sure
if this is always a safe thing to do.

When we gain a new "command" in the protocol, if we just read it
without understanding it, would we open ourselves to a risk of
breaking the protocol communication, worst of which may be to
deadlock?  A new command, when received by a more recent helper that
understands how to react to it, may _require_ it to write more than
"username" and "password" back to "git" from get_credential(), for
example, but the helper with this patch alone, while not complaining
about seeing such a new and unknown command, would not know how to
compute and write that third thing other than "username" and
"password"---would the other side who issued that new command get
stuck waiting for us to return the third thing?  Worse yet, the new
command may expect us to read further in get_credential()
(e.g. maybe they will give us a challenge, which may need to be used
when yielding the "username" and "password" things), but because we
do not even know we need to read, their attempt to write to us may
get them stuck, and that is when we are expecting to write to them,
easily leading to a deadlock, no?

> All of the other helpers (store, cache, osxkeychain, libsecret,
> gnome-keyring) except `netrc` currently ignore any credential lines
> that are not recognised, whereas the Windows helper (wincred) instead
> dies.

Is that different from saying "everybody other than netrc and win
ignore unknown"?

> Fix the discrepancy and ignore unknown lines in the wincred helper.

OK.  As long as everybody consistently ignores, and possibly opens
themselves up consistently to a protocol mismatch issue, it is OK.
We will know if that can be a real problem or does not happen in
practice.

> Signed-off-by: Matthew John Cheetham <mjcheetham@outlook.com>
> ---
>  contrib/credential/wincred/git-credential-wincred.c | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/contrib/credential/wincred/git-credential-wincred.c b/contrib/credential/wincred/git-credential-wincred.c
> index 5091048f9c6..ead6e267c78 100644
> --- a/contrib/credential/wincred/git-credential-wincred.c
> +++ b/contrib/credential/wincred/git-credential-wincred.c
> @@ -278,8 +278,11 @@ static void read_credential(void)
>  			wusername = utf8_to_utf16_dup(v);
>  		} else if (!strcmp(buf, "password"))
>  			password = utf8_to_utf16_dup(v);
> -		else
> -			die("unrecognized input");
> +		/*
> +		 * Ignore other lines; we don't know what they mean, but
> +		 * this future-proofs us when later versions of git do
> +		 * learn new lines, and the helpers are updated to match.
> +		 */
>  	}
>  }

Re: [PATCH 1/3] wincred: ignore unknown lines (do not die)

[Jeff King]

On Thu, Sep 22, 2022 at 02:19:43PM -0700, Junio C Hamano wrote:

> > It is the expectation that credential helpers be liberal in what they
> > accept and conservative in what they return, to allow for future growth
> > and evolution of the protocol/interaction.
> 
> That is nice in principle, and the updated code below may work well
> with existing "other side of the connection" (codepaths in "git"
> that asks credential API to talk to the helpers), but I am not sure
> if this is always a safe thing to do.
> 
> When we gain a new "command" in the protocol, if we just read it
> without understanding it, would we open ourselves to a risk of
> breaking the protocol communication, worst of which may be to
> deadlock?  A new command, when received by a more recent helper that
> understands how to react to it, may _require_ it to write more than
> "username" and "password" back to "git" from get_credential(), for
> example, but the helper with this patch alone, while not complaining
> about seeing such a new and unknown command, would not know how to
> compute and write that third thing other than "username" and
> "password"---would the other side who issued that new command get
> stuck waiting for us to return the third thing?  Worse yet, the new
> command may expect us to read further in get_credential()
> (e.g. maybe they will give us a challenge, which may need to be used
> when yielding the "username" and "password" things), but because we
> do not even know we need to read, their attempt to write to us may
> get them stuck, and that is when we are expecting to write to them,
> easily leading to a deadlock, no?

This open-endedness was an intentional part of the original
credential-helper design. Helpers are always "best effort", and it is OK
if they ignore a request, or return a partial result. If the sending
side really wants to extend the protocol in a way that the other side
doesn't act at all (say, they "username" to be used _only_ if the helper
also understands a new "foobar" key), then they should be adding the new
fields as an atomic unit. I.e., "foobar", "foobar-userame", and so on.
And then existing helpers which don't understand the new feature will
just ignore it totally.

In practice, I think it's not that big a deal either way, because the
setup of these helpers is under the control of the user. So if you
really want to use "foobar" but a helper doesn't support it, then you'd
just remove that helper from your config.

-Peff

[PATCH 2/3] netrc: ignore unknown lines (do not die)

[Matthew John Cheetham via GitGitGadget]

From: Matthew John Cheetham <mjcheetham@outlook.com>

Contrary to the documentation on credential helpers, as well as the help
text for git-credential-netrc itself, this helper will `die` when
presented with an unknown property/attribute/token.

Correct the behaviour here by skipping and ignoring any tokens that are
unknown. This means all helpers in the tree are consistent and ignore
any unknown credential properties/attributes.

Signed-off-by: Matthew John Cheetham <mjcheetham@outlook.com>
---
 contrib/credential/netrc/git-credential-netrc.perl | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/contrib/credential/netrc/git-credential-netrc.perl b/contrib/credential/netrc/git-credential-netrc.perl
index bc57cc65884..9fb998ae090 100755
--- a/contrib/credential/netrc/git-credential-netrc.perl
+++ b/contrib/credential/netrc/git-credential-netrc.perl
@@ -356,7 +356,10 @@ sub read_credential_data_from_stdin {
 		next unless m/^([^=]+)=(.+)/;
 
 		my ($token, $value) = ($1, $2);
-		die "Unknown search token $token" unless exists $q{$token};
+
+		# skip any unknown tokens
+		next unless exists $q{$token};
+
 		$q{$token} = $value;
 		log_debug("We were given search token $token and value $value");
 	}
-- 
gitgitgadget

[PATCH 3/3] osxkeychain: clarify that we ignore unknown lines

[Matthew John Cheetham via GitGitGadget]

From: Matthew John Cheetham <mjcheetham@outlook.com>

Like in all the other credential helpers, the osxkeychain helper
ignores unknown credential lines.

Add a comment (a la the other helpers) to make it clear and explicit
that this is the desired behaviour.

Signed-off-by: Matthew John Cheetham <mjcheetham@outlook.com>
---
 contrib/credential/osxkeychain/git-credential-osxkeychain.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/contrib/credential/osxkeychain/git-credential-osxkeychain.c b/contrib/credential/osxkeychain/git-credential-osxkeychain.c
index bf77748d602..e29cc28779d 100644
--- a/contrib/credential/osxkeychain/git-credential-osxkeychain.c
+++ b/contrib/credential/osxkeychain/git-credential-osxkeychain.c
@@ -159,6 +159,11 @@ static void read_credential(void)
 			username = xstrdup(v);
 		else if (!strcmp(buf, "password"))
 			password = xstrdup(v);
+		/*
+		 * Ignore other lines; we don't know what they mean, but
+		 * this future-proofs us when later versions of git do
+		 * learn new lines, and the helpers are updated to match.
+		 */
 	}
 }
 
-- 
gitgitgadget

Re: [PATCH 0/3] Correct credential helper discrepancies handling input

[Derrick Stolee]

On 9/22/2022 12:59 PM, Matthew John Cheetham via GitGitGadget wrote:
> Whilst working in the credential helper and auth space I happened to notice
> that the behaviour of some of the credential helpers in contrib/credential/
> is not consistent.
> 
> Specifically both the git-credential-wincred and git-credential-netrc
> helpers die when they receive unknown credential property keys, which is
> contrary to the behaviour of all the other in-tree helpers including:
> git-credential-cache, -store, -libsecret, -gnome-keyring, -osxkeychain.
> 
> Also update the git-credential-osxkeychain helper to include a comment
> making it's behaviour explicit in ignoring unknown keys, as per other
> helpers.

Thanks for sending this patches separate from your earlier RFC. I
think these patches speak for themselves and are good to go.

Thanks,
-Stolee

Re: [PATCH 0/3] Correct credential helper discrepancies handling input

[Jeff King]

On Thu, Sep 22, 2022 at 04:59:31PM +0000, Matthew John Cheetham via GitGitGadget wrote:

> Whilst working in the credential helper and auth space I happened to notice
> that the behaviour of some of the credential helpers in contrib/credential/
> is not consistent.
> 
> Specifically both the git-credential-wincred and git-credential-netrc
> helpers die when they receive unknown credential property keys, which is
> contrary to the behaviour of all the other in-tree helpers including:
> git-credential-cache, -store, -libsecret, -gnome-keyring, -osxkeychain.
> 
> Also update the git-credential-osxkeychain helper to include a comment
> making it's behaviour explicit in ignoring unknown keys, as per other
> helpers.

Thanks, I think these are all sensible changes. The new behavior matches
the original vision of the helper protocol. But more importantly, it
brings them in line with all of the _other_ existing helpers. Which is
no coincidence, since the designer of the protocol wrote most of them. ;)

I haven't had a chance to look carefully at the rest of your larger
series yet, but I think these are a nice standalone improvement
regardless of what happens there.

-Peff