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[PATCH 10/10] add UNLEAK annotation for reducing leak false positives




It's a common pattern in git commands to allocate some
memory that should last for the lifetime of the program and
then not bother to free it, relying on the OS to throw it
away.

This keeps the code simple, and it's fast (we don't waste
time traversing structures or calling free at the end of the
program). But it also triggers warnings from memory-leak
checkers like valgrind or LSAN. They know that the memory
was still allocated at program exit, but they don't know
_when_ the leaked memory stopped being useful. If it was
early in the program, then it's probably a real and
important leak. But if it was used right up until program
exit, it's not an interesting leak and we'd like to suppress
it so that we can see the real leaks.

This patch introduces an UNLEAK() macro that lets us do so.
To understand its design, let's first look at some of the
alternatives.

Unfortunately the suppression systems offered by
leak-checking tools don't quite do what we want. A
leak-checker basically knows two things:

  1. Which blocks were allocated via malloc, and the
     callstack during the allocation.

  2. Which blocks were left un-freed at the end of the
     program (and which are unreachable, but more on that
     later).

Their suppressions work by mentioning the function or
callstack of a particular allocation, and marking it as OK
to leak.  So imagine you have code like this:

  int main(void)
  {
	/* this allocates some memory */
	char *p = some_function();
	printf("%s", p);
	return 0;
  }

You can say "ignore allocations from some_function(),
they're not leaks". But that's not right. That function may
be called elsewhere, too, and we would potentially want to
know about those leaks.

So you can say "ignore the callstack when main calls
some_function".  That works, but your annotations are
brittle. In this case it's only two functions, but you can
imagine that the actual allocation is much deeper. If any of
the intermediate code changes, you have to update the
suppression.

What we _really_ want to say is that "the value assigned to
p at the end of the function is not a real leak". But
leak-checkers can't understand that; they don't know about
"p" in the first place.

However, we can do something a little bit tricky if we make
some assumptions about how leak-checkers work. They
generally don't just report all un-freed blocks. That would
report even globals which are still accessible when the
leak-check is run.  Instead they take some set of memory
(like BSS) as a root and mark it as "reachable". Then they
scan the reachable blocks for anything that looks like a
pointer to a malloc'd block, and consider that block
reachable. And then they scan those blocks, and so on,
transitively marking anything reachable from a global as
"not leaked" (or at least leaked in a different category).

So we can mark the value of "p" as reachable by putting it
into a variable with program lifetime. One way to do that is
to just mark "p" as static. But that actually affects the
run-time behavior if the function is called twice (you
aren't likely to call main() twice, but some of our cmd_*()
functions are called from other commands).

Instead, we can trick the leak-checker by putting the value
into _any_ reachable bytes. This patch keeps a global
linked-list of bytes copied from "unleaked" variables. That
list is reachable even at program exit, which confers
recursive reachability on whatever values we unleak.

In other words, you can do:

  int main(void)
  {
	char *p = some_function();
	printf("%s", p);
	UNLEAK(p);
	return 0;
  }

to annotate "p" and suppress the leak report.

But wait, couldn't we just say "free(p)"? In this toy
example, yes. But using UNLEAK() has several advantages over
actually freeing the memory:

  1. It can be compiled conditionally. There's no need in
     normal runs to do this free(), and it just wastes time.
     By using a macro, we can get the benefit for leak-check
     builds with zero cost for normal builds (this patch
     uses a compile-time check, though we could clearly also
     make it a run-time check at very low cost).

     Of course one could also hide free() behind a macro, so
     this is really just arguing for having UNLEAK(), not
     for its particular implementation.

  2. It's recursive across structures. In many cases our "p"
     is not just a pointer, but a complex struct whose
     fields may have been allocated by a sub-function. And
     in some cases (e.g., dir_struct) we don't even have a
     function which knows how to free all of the struct
     members.

     By marking the struct itself as reachable, that confers
     reachability on any pointers it contains (including those
     found in embedded structs, or reachable by walking
     heap blocks recursively.

  3. It works on cases where we're not sure if the value is
     allocated or not. For example:

       char *p = argc > 1 ? argv[1] : some_function();

     It's safe to use UNLEAK(p) here, because it's not
     freeing any memory. In the case that we're pointing to
     argv here, the reachability checker will just ignore
     our bytes.

  4. Because it's not actually freeing memory, you can
     UNLEAK() before we are finished accessing the variable.
     This is helpful in cases like this:

       char *p = some_function();
       return another_function(p);

     Writing this with free() requires:

       int ret;
       char *p = some_function();
       ret = another_function(p);
       free(p);
       return ret;

     But with unleak we can just write:

       char *p = some_function();
       UNLEAK(p);
       return another_function(p);

This patch adds the UNLEAK() macro and enables it
automatically when Git is compiled with SANITIZE=leak.
It adds some UNLEAK() annotations to show off how the
feature works. On top of other recent leak fixes, these are
enough to get t0000 and t0001 to pass when compiled with
LSAN.

Signed-off-by: Jeff King <peff@xxxxxxxx>
---
 Makefile           |  3 +++
 builtin/add.c      |  2 ++
 builtin/commit.c   |  1 +
 builtin/config.c   |  4 ++++
 builtin/init-db.c  |  2 ++
 builtin/ls-files.c |  1 +
 builtin/worktree.c |  2 ++
 git-compat-util.h  |  7 +++++++
 usage.c            | 13 +++++++++++++
 9 files changed, 35 insertions(+)

diff --git a/Makefile b/Makefile
index f2bb7f2f63..c052f09bba 100644
--- a/Makefile
+++ b/Makefile
@@ -1036,6 +1036,9 @@ BASIC_CFLAGS += -fno-omit-frame-pointer
 ifneq ($(filter undefined,$(SANITIZERS)),)
 BASIC_CFLAGS += -DNO_UNALIGNED_LOADS
 endif
+ifneq ($(filter leak,$(SANITIZERS)),)
+BASIC_CFLAGS += -DSUPPRESS_ANNOTATED_LEAKS
+endif
 endif
 
 ifndef sysconfdir
diff --git a/builtin/add.c b/builtin/add.c
index ef625e3fb8..a648cf4c56 100644
--- a/builtin/add.c
+++ b/builtin/add.c
@@ -515,5 +515,7 @@ int cmd_add(int argc, const char **argv, const char *prefix)
 			die(_("Unable to write new index file"));
 	}
 
+	UNLEAK(pathspec);
+	UNLEAK(dir);
 	return exit_status;
 }
diff --git a/builtin/commit.c b/builtin/commit.c
index b3b04f5dd3..de775d906c 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -1819,5 +1819,6 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 		print_summary(prefix, &oid, !current_head);
 
 	strbuf_release(&err);
+	UNLEAK(sb);
 	return 0;
 }
diff --git a/builtin/config.c b/builtin/config.c
index 52a4606243..d13daeeb55 100644
--- a/builtin/config.c
+++ b/builtin/config.c
@@ -631,6 +631,7 @@ int cmd_config(int argc, const char **argv, const char *prefix)
 		check_write();
 		check_argc(argc, 2, 2);
 		value = normalize_value(argv[0], argv[1]);
+		UNLEAK(value);
 		ret = git_config_set_in_file_gently(given_config_source.file, argv[0], value);
 		if (ret == CONFIG_NOTHING_SET)
 			error(_("cannot overwrite multiple values with a single value\n"
@@ -641,6 +642,7 @@ int cmd_config(int argc, const char **argv, const char *prefix)
 		check_write();
 		check_argc(argc, 2, 3);
 		value = normalize_value(argv[0], argv[1]);
+		UNLEAK(value);
 		return git_config_set_multivar_in_file_gently(given_config_source.file,
 							      argv[0], value, argv[2], 0);
 	}
@@ -648,6 +650,7 @@ int cmd_config(int argc, const char **argv, const char *prefix)
 		check_write();
 		check_argc(argc, 2, 2);
 		value = normalize_value(argv[0], argv[1]);
+		UNLEAK(value);
 		return git_config_set_multivar_in_file_gently(given_config_source.file,
 							      argv[0], value,
 							      CONFIG_REGEX_NONE, 0);
@@ -656,6 +659,7 @@ int cmd_config(int argc, const char **argv, const char *prefix)
 		check_write();
 		check_argc(argc, 2, 3);
 		value = normalize_value(argv[0], argv[1]);
+		UNLEAK(value);
 		return git_config_set_multivar_in_file_gently(given_config_source.file,
 							      argv[0], value, argv[2], 1);
 	}
diff --git a/builtin/init-db.c b/builtin/init-db.c
index 47823f9aa4..c9b7946bad 100644
--- a/builtin/init-db.c
+++ b/builtin/init-db.c
@@ -579,6 +579,8 @@ int cmd_init_db(int argc, const char **argv, const char *prefix)
 			set_git_work_tree(work_tree);
 	}
 
+	UNLEAK(real_git_dir);
+
 	flags |= INIT_DB_EXIST_OK;
 	return init_db(git_dir, real_git_dir, template_dir, flags);
 }
diff --git a/builtin/ls-files.c b/builtin/ls-files.c
index e1339e6d17..8c713c47ac 100644
--- a/builtin/ls-files.c
+++ b/builtin/ls-files.c
@@ -673,5 +673,6 @@ int cmd_ls_files(int argc, const char **argv, const char *cmd_prefix)
 		return bad ? 1 : 0;
 	}
 
+	UNLEAK(dir);
 	return 0;
 }
diff --git a/builtin/worktree.c b/builtin/worktree.c
index c98e2ce5f5..de26849f55 100644
--- a/builtin/worktree.c
+++ b/builtin/worktree.c
@@ -381,6 +381,8 @@ static int add(int ac, const char **av, const char *prefix)
 		branch = opts.new_branch;
 	}
 
+	UNLEAK(path);
+	UNLEAK(opts);
 	return add_worktree(path, branch, &opts);
 }
 
diff --git a/git-compat-util.h b/git-compat-util.h
index 6678b488cc..01cde2e375 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -1169,4 +1169,11 @@ static inline int is_missing_file_error(int errno_)
 
 extern int cmd_main(int, const char **);
 
+#ifdef SUPPRESS_ANNOTATED_LEAKS
+extern void unleak_memory(const void *ptr, size_t len);
+#define UNLEAK(var) unleak_memory(&(var), sizeof(var));
+#else
+#define UNLEAK(var)
+#endif
+
 #endif
diff --git a/usage.c b/usage.c
index 1ea7df9a20..780ed73be6 100644
--- a/usage.c
+++ b/usage.c
@@ -241,3 +241,16 @@ NORETURN void BUG(const char *fmt, ...)
 	va_end(ap);
 }
 #endif
+
+void unleak_memory(const void *ptr, size_t len)
+{
+	static struct suppressed_leak_root {
+		struct suppressed_leak_root *next;
+		char data[FLEX_ARRAY];
+	} *suppressed_leaks;
+	struct suppressed_leak_root *root;
+
+	FLEX_ALLOC_MEM(root, data, ptr, len);
+	root->next = suppressed_leaks;
+	suppressed_leaks = root;
+}
-- 
2.14.1.721.gc5bc1565f1