[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@peff.net>
---
 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(+)