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[v2,09/13] merge-ort: avoid accidental API mis-use

Message ID becd451030183e6271646a4bf006c2466822326e.1620094339.git.gitgitgadget@gmail.com (mailing list archive)
State Superseded
Headers show
Series Optimization batch 11: avoid repeatedly detecting same renames | expand

Commit Message

Elijah Newren May 4, 2021, 2:12 a.m. UTC 
From: Elijah Newren <newren@gmail.com>

Previously, callers of the merge-ort API could have passed an
uninitialized value for struct merge_result *result.  However, we want
to check result to see if it has cached renames from a previous merge
that we can reuse; such values would be found behind result->priv.
However, if result->priv is uninitialized, attempting to access behind
it will give a segfault.  So, we need result->priv to be NULL (which
will be the case if the caller does a memset(&result, 0)), or be written
by a previous call to the merge-ort machinery.  Documenting this
requirement may help, but despite being the person who introduced this
requirement, I still missed it once and it did not fail in a very clear
way and led to a long debugging session.

Add a _properly_initialized field to merge_result; that value will be
0 if the caller zero'ed the merge_result, it will be set to a very
specific value by a previous run by the merge-ort machinery, and if it's
uninitialized it will most likely either be 0 or some value that does
not match the specific one we'd expect allowing us to throw a much more
meaningful error.

Signed-off-by: Elijah Newren <newren@gmail.com>
---
 merge-ort.c | 7 +++++++
 merge-ort.h | 2 ++
 2 files changed, 9 insertions(+)

Comments

Derrick Stolee May 17, 2021, 2:10 p.m. UTC | #1 
On 5/3/21 10:12 PM, Elijah Newren via GitGitGadget wrote:
> From: Elijah Newren <newren@gmail.com>
> 
> Previously, callers of the merge-ort API could have passed an
> uninitialized value for struct merge_result *result.  However, we want
> to check result to see if it has cached renames from a previous merge
> that we can reuse; such values would be found behind result->priv.
> However, if result->priv is uninitialized, attempting to access behind
> it will give a segfault.  So, we need result->priv to be NULL (which
> will be the case if the caller does a memset(&result, 0)), or be written
> by a previous call to the merge-ort machinery.  Documenting this
> requirement may help, but despite being the person who introduced this
> requirement, I still missed it once and it did not fail in a very clear
> way and led to a long debugging session.

I empathize with your frustration here.

> Add a _properly_initialized field to merge_result; that value will be
> 0 if the caller zero'ed the merge_result, it will be set to a very
> specific value by a previous run by the merge-ort machinery, and if it's
> uninitialized it will most likely either be 0 or some value that does
> not match the specific one we'd expect allowing us to throw a much more
> meaningful error.

This approach is an interesting pattern to follow for future
efforts, too.

> +static unsigned RESULT_INITIALIZED = 0x1abe11ed; /* unlikely accidental value */
> +
>  struct traversal_callback_data {
>  	unsigned long mask;
>  	unsigned long dirmask;
> @@ -3746,6 +3748,10 @@ static void merge_start(struct merge_options *opt, struct merge_result *result)
>  	assert(opt->obuf.len == 0);
>  
>  	assert(opt->priv == NULL);
> +	if (result->_properly_initialized != 0 &&
> +	    result->_properly_initialized != RESULT_INITIALIZED)
> +		BUG("struct merge_result passed to merge_incore_*recursive() must be zeroed or filled with values from a previous run");
> +	assert(!!result->priv == !!result->_properly_initialized);
>  	if (result->priv) {
>  		opt->priv = result->priv;
>  		result->priv = NULL;
> @@ -3905,6 +3911,7 @@ static void merge_ort_nonrecursive_internal(struct merge_options *opt,
>  	result->clean &= strmap_empty(&opt->priv->conflicted);
>  	if (!opt->priv->call_depth) {
>  		result->priv = opt->priv;
> +		result->_properly_initialized = RESULT_INITIALIZED;
>  		opt->priv = NULL;
>  	}
>  }
> diff --git a/merge-ort.h b/merge-ort.h
> index d53a0a339f33..c011864ffeb1 100644
> --- a/merge-ort.h
> +++ b/merge-ort.h
> @@ -29,6 +29,8 @@ struct merge_result {
>  	 * !clean) and to print "CONFLICT" messages.  Not for external use.
>  	 */
>  	void *priv;
> +	/* Also private */
> +	unsigned _properly_initialized;
>  };

Will work as intended. Thank you for reducing developer friction.

Thanks,
-Stolee
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Patch

diff --git a/merge-ort.c b/merge-ort.c
index a342cc6344fd..e6a02fa928f5 100644
--- a/merge-ort.c
+++ b/merge-ort.c
@@ -53,6 +53,8 @@  enum merge_side {
 	MERGE_SIDE2 = 2
 };
 
+static unsigned RESULT_INITIALIZED = 0x1abe11ed; /* unlikely accidental value */
+
 struct traversal_callback_data {
 	unsigned long mask;
 	unsigned long dirmask;
@@ -3746,6 +3748,10 @@  static void merge_start(struct merge_options *opt, struct merge_result *result)
 	assert(opt->obuf.len == 0);
 
 	assert(opt->priv == NULL);
+	if (result->_properly_initialized != 0 &&
+	    result->_properly_initialized != RESULT_INITIALIZED)
+		BUG("struct merge_result passed to merge_incore_*recursive() must be zeroed or filled with values from a previous run");
+	assert(!!result->priv == !!result->_properly_initialized);
 	if (result->priv) {
 		opt->priv = result->priv;
 		result->priv = NULL;
@@ -3905,6 +3911,7 @@  static void merge_ort_nonrecursive_internal(struct merge_options *opt,
 	result->clean &= strmap_empty(&opt->priv->conflicted);
 	if (!opt->priv->call_depth) {
 		result->priv = opt->priv;
+		result->_properly_initialized = RESULT_INITIALIZED;
 		opt->priv = NULL;
 	}
 }
diff --git a/merge-ort.h b/merge-ort.h
index d53a0a339f33..c011864ffeb1 100644
--- a/merge-ort.h
+++ b/merge-ort.h
@@ -29,6 +29,8 @@  struct merge_result {
 	 * !clean) and to print "CONFLICT" messages.  Not for external use.
 	 */
 	void *priv;
+	/* Also private */
+	unsigned _properly_initialized;
 };
 
 /*