[PATCH v3 09/11] serve: reject bogus v2 "command=ls-refs=foo"

When we see a line from the client like "command=ls-refs", we parse
everything after the equals sign as a capability, which we check against
our capabilities table. If we don't recognize the command (e.g.,
"command=foo"), we'll reject it.

But in parse_command(), we use the same get_capability() parser for
parsing non-command lines. So if we see "command=ls-refs=foo", we will
feed "ls-refs=foo" to get_capability(), which will say "OK, that's
ls-refs, with value 'foo'". But then we simply ignore the value
entirely.

The client is violating the spec here, which says:

      command = PKT-LINE("command=" key LF)
      key = 1*(ALPHA | DIGIT | "-_")

I.e., the key is not even allowed to have an equals sign in it. Whereas
a real non-command capability does allow a value:

      capability = PKT-LINE(key[=value] LF)

So by reusing the same get_capability() parser, we are mixing up the
"key" and "capability" tokens. However, since that parser tells us
whether it saw an "=", we can still use it; we just need to reject any
input that produces a non-NULL value field.

The current behavior isn't really hurting anything (the client should
never send such a request, and if it does, we just ignore the "value"
part). But since it does violate the spec, let's tighten it up to
prevent any surprising behavior.

Signed-off-by: Jeff King <redacted>
---
 serve.c              |  2 +-
 t/t5701-git-serve.sh | 10 ++++++++++
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/serve.c b/serve.c
index 1a7c8a118f..db5ecfed2d 100644
--- a/serve.c
+++ b/serve.c

@@ -220,7 +220,7 @@ static int parse_command(const char *key, struct protocol_capability **command)
 		if (*command)
 			die("command '%s' requested after already requesting command '%s'",
 			    out, (*command)->name);
-		if (!cmd || !cmd->advertise(the_repository, NULL) || !cmd->command)
+		if (!cmd || !cmd->advertise(the_repository, NULL) || !cmd->command || value)
 			die("invalid command '%s'", out);
 
 		*command = cmd;

diff --git a/t/t5701-git-serve.sh b/t/t5701-git-serve.sh
index 520672f842..2e51886def 100755
--- a/t/t5701-git-serve.sh
+++ b/t/t5701-git-serve.sh

@@ -72,6 +72,16 @@ test_expect_success 'request invalid command' '
 	test_i18ngrep "invalid command" err
 '
 
+test_expect_success 'requested command is command=value' '
+	test-tool pkt-line pack >in <<-EOF &&
+	command=ls-refs=whatever
+	object-format=$(test_oid algo)
+	0000
+	EOF
+	test_must_fail test-tool serve-v2 --stateless-rpc 2>err <in &&
+	grep invalid.command.*ls-refs=whatever err
+'
+
 test_expect_success 'wrong object-format' '
 	test-tool pkt-line pack >in <<-EOF &&
 	command=fetch

-- 
2.33.0.917.g33ebf6a5f6