[PATCH 0/2] fix a segfault in get_main_ref_store()

[PATCH 0/2] fix a segfault in get_main_ref_store()

[Jeff King]

I stumbled across a BUG() today. But interestingly, in the current tip
of master it actually segfaults instead! This fixes the segfault (back
into a BUG(), and then fixes the caller to avoid the BUG() in the first
place).

  [1/2]: get_main_ref_store: BUG() when outside a repository
  [2/2]: config: die when --blob is used outside a repository

 builtin/config.c       | 3 +++
 refs.c                 | 3 +++
 t/t1307-config-blob.sh | 4 ++++
 3 files changed, 10 insertions(+)

-Peff

[PATCH 1/2] get_main_ref_store: BUG() when outside a repository

[Jeff King]

If we don't have a repository, then we can't initialize the
ref store.  Prior to 64a741619d (refs: store the main ref
store inside the repository struct, 2018-04-11), we'd try to
access get_git_dir(), and outside a repository that would
trigger a BUG(). After that commit, though, we directly use
the_repository->git_dir; if it's NULL we'll just segfault.

Let's catch this case and restore the BUG() behavior.
Obviously we don't ever want to hit this code, but a BUG()
is a lot more helpful than a segfault if we do.

Signed-off-by: Jeff King <peff@peff.net>
---
 refs.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/refs.c b/refs.c
index 64aadd14c9..2e4a42f459 100644
--- a/refs.c
+++ b/refs.c
@@ -1668,6 +1668,9 @@ struct ref_store *get_main_ref_store(struct repository *r)
 	if (r->refs)
 		return r->refs;
 
+	if (!r->gitdir)
+		BUG("attempting to get main_ref_store outside of repository");
+
 	r->refs = ref_store_init(r->gitdir, REF_STORE_ALL_CAPS);
 	return r->refs;
 }
-- 
2.17.0.1052.g7d69f75dbf

[PATCH 2/2] config: die when --blob is used outside a repository

[Jeff King]

If you run "config --blob" outside of a repository, then we
eventually try to resolve the blob name and hit a BUG().
Let's catch this earlier and provide a useful message.

Note that we could also catch this much lower in the stack,
in git_config_from_blob_ref(). That might cover other
callsites, too, but it's unclear whether those ones would
actually be bugs or not. So let's leave the low-level
functions to assume the caller knows what it's doing (and
BUG() if it turns out it doesn't).

Signed-off-by: Jeff King <peff@peff.net>
---
 builtin/config.c       | 3 +++
 t/t1307-config-blob.sh | 4 ++++
 2 files changed, 7 insertions(+)

diff --git a/builtin/config.c b/builtin/config.c
index 69e7270356..4155734f4a 100644
--- a/builtin/config.c
+++ b/builtin/config.c
@@ -602,6 +602,9 @@ int cmd_config(int argc, const char **argv, const char *prefix)
 	if (use_local_config && nongit)
 		die(_("--local can only be used inside a git repository"));
 
+	if (given_config_source.blob && nongit)
+		die(_("--blob can only be used inside a git repository"));
+
 	if (given_config_source.file &&
 			!strcmp(given_config_source.file, "-")) {
 		given_config_source.file = NULL;
diff --git a/t/t1307-config-blob.sh b/t/t1307-config-blob.sh
index eed31ffa30..37dc689d8c 100755
--- a/t/t1307-config-blob.sh
+++ b/t/t1307-config-blob.sh
@@ -73,4 +73,8 @@ test_expect_success 'can parse blob ending with CR' '
 	test_cmp expect actual
 '
 
+test_expect_success 'config --blob outside of a repository is an error' '
+	test_must_fail nongit git config --blob=foo --list
+'
+
 test_done
-- 
2.17.0.1052.g7d69f75dbf

Re: [PATCH 2/2] config: die when --blob is used outside a repository

[Taylor Blau]

On Fri, May 18, 2018 at 03:27:04PM -0700, Jeff King wrote:
> If you run "config --blob" outside of a repository, then we
> eventually try to resolve the blob name and hit a BUG().
> Let's catch this earlier and provide a useful message.

I think that this approach is sensible. Let's (1) fix a SIGSEGV that
should be a BUG(), and (2) make sure that we never get there in the
first place.

This looks fine to me.

> Note that we could also catch this much lower in the stack,
> in git_config_from_blob_ref(). That might cover other
> callsites, too, but it's unclear whether those ones would
> actually be bugs or not. So let's leave the low-level
> functions to assume the caller knows what it's doing (and
> BUG() if it turns out it doesn't).
>
> Signed-off-by: Jeff King <peff@peff.net>

Thanks,
Taylor

Re: [PATCH 1/2] get_main_ref_store: BUG() when outside a repository

[Stefan Beller]

On Fri, May 18, 2018 at 3:25 PM, Jeff King <peff@peff.net> wrote:
> If we don't have a repository, then we can't initialize the
> ref store.  Prior to 64a741619d (refs: store the main ref
> store inside the repository struct, 2018-04-11), we'd try to
> access get_git_dir(), and outside a repository that would
> trigger a BUG(). After that commit, though, we directly use
> the_repository->git_dir; if it's NULL we'll just segfault.
>
> Let's catch this case and restore the BUG() behavior.
> Obviously we don't ever want to hit this code, but a BUG()
> is a lot more helpful than a segfault if we do.

That is true and an immediate bandaid; an alternative would
be to do:

  if (!r->gitdir)
    /* not in a git directory ? */
    return NULL;
    /* We signal the caller the absence of a git repo by NULL ness
      of the ref store */

However that we would need to catch at all callers of
get_main_ref_store and error out accordingly.

Then the trade off would be, how many callers to the main ref
store do we have compared to options that rely on a ref store
present? (I assume a patch like the second patch would show
up in more cases now for all BUGs that we discover via this patch.
The tradeoff is just if we want to report all these early by checking
the config and system state, or wait until we get NULL ref store
and then bail)

I think checking early makes sense; I am not so sure about this
patch; for the time being it makes sense, though in the long run,
we rather want to catch this at a higher level:

r->gitdir is supposedly never NULL, so we shall not
produce this state. Maybe we want to set the_repository to NULL
if set_git_dir fails (via repo_set_gitdir in setup_git_env, which both
return void today).

Enough of my rambling, the patches look good!
Stefan

Re: [PATCH 0/2] fix a segfault in get_main_ref_store()

[Johannes Schindelin]

Hi Peff,

On Fri, 18 May 2018, Jeff King wrote:

> I stumbled across a BUG() today. But interestingly, in the current tip
> of master it actually segfaults instead! This fixes the segfault (back
> into a BUG(), and then fixes the caller to avoid the BUG() in the first
> place).
> 
>   [1/2]: get_main_ref_store: BUG() when outside a repository
>   [2/2]: config: die when --blob is used outside a repository
> 
>  builtin/config.c       | 3 +++
>  refs.c                 | 3 +++
>  t/t1307-config-blob.sh | 4 ++++
>  3 files changed, 10 insertions(+)

Both patches look obviously correct to me.

Thanks,
Dscho