Re: [PATCH v3 04/11] unpack-trees: introduce preserve_ignored to unpack_trees_options

On Sat, Oct 02 2021, Ævar Arnfjörð Bjarmason wrote:

On Fri, Oct 01 2021, Elijah Newren wrote:

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On Fri, Oct 1, 2021 at 1:47 AM Ævar Arnfjörð Bjarmason [off-list ref] wrote:

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On Thu, Sep 30 2021, Elijah Newren wrote:

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On Thu, Sep 30, 2021 at 7:15 AM Ævar Arnfjörð Bjarmason
[off-list ref] wrote:

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On Wed, Sep 29 2021, Elijah Newren wrote:

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I might be going on a tangent here, but looking at that patch, I'm
worried that dir_init() was buggy and that you perpetuated that bug
with DIR_INIT.  Note that dir_struct has a struct strbuf basebuf
member, which neither dir_init() or DIR_INIT initialize properly
(using either strbuf_init() or STRBUF_INIT).  As far as I can tell,
dir.c relies on either strbuf_add() calls to just happen to work with
this incorrectly initialized strbuf, or else use the strbuf_init()
call in prep_exclude() to do so, using the following snippet:

    if (!dir->basebuf.buf)
        strbuf_init(&dir->basebuf, PATH_MAX);

However, earlier in that same function we see

    if (stk->baselen <= baselen &&
        !strncmp(dir->basebuf.buf, base, stk->baselen))
            break;

So either that function can never have dir->basebuf.buf be NULL and
the strbuf_init() is dead code, or else it's possible for us to
trigger a segfault.  If it's the former, it may just be a ticking time
bomb that will transform into the latter with some other change,
because it's not at all obvious to me how dir->basebuf gets
initialized appropriately to avoid that strncmp call.  Perhaps there
is some invariant where exclude_stack is only set up by previous calls
to prep_exclude() and those won't set up exclude_stack until first
initializing basebuf.  But that really at least deserves a comment
about how we're abusing basebuf, and would probably be cleaner if we
initialized basebuf to STRBUF_INIT.

...because yes, I forgot about that when sending you the diff-on-top,
sorry. Yes that's buggy with the diff-on-top I sent you.

That bug didn't come from the diff-on-top you sent me, it came from
the commit already merged to master -- ce93a4c6127  (dir.[ch]: replace
dir_init() with DIR_INIT, 2021-07-01), merged as part of
ab/struct-init on Jul 16.

Ah, I misunderstood you there. I'll look at that / fix it. Sorry.

Just to tie up this loose end: Yes this control flow suck, and I've got
some patches to unpack-trees.[ch] & dir.[ch] I'm about to submit to fix
it. But just to comment on the existing behavior of the code, i.e. your
(above):

    "So either that function can never have dir->basebuf.buf be NULL and
    the strbuf_init() is dead code, or else it's possible for us to
    trigger a segfault.".

I hadn't had time to look into it when I said I'd fix it, but now that I
have I found thath there's nothing to fix, and this code wasn't buggy
either before or after my ce93a4c6127 (dir.[ch]: replace dir_init() with
DIR_INIT, 2021-07-01). I.e. we do have the invariant you mentioned.

The dir.[ch] API has always relied on the "struct dir_struct" being
zero'd out. First with memset() before your eceba532141 (dir: fix
problematic API to avoid memory leaks, 2020-08-18), and after my
ce93a4c6127 with the DIR_INIT, which both amount to the same thing.

We both missed a caller that used neither dir_init() nor uses DIR_INIT
now, but it uses "{ 0 }", so it's always zero'd.

Now, of course it being zero'd *would* segfault if you feed
"dir->basebuf.buf" to strncmp() as you note above, but that code isn't
reachable. The structure of that function is (pseudocode):

void prep_exclude(...)
{
	struct exclude_stack *stk = NULL;
	[...]

	while ((stk = dir->exclude_stack) != NULL)
		/* the strncmp() against "dir->basebuf.buf" is here */

	/* maybe we'll early return here */

	if (!dir->basebuf.buf)
		strbuf_init(&dir->basebuf, PATH_MAX);

	/*
         * Code that sets dir->exclude_stack to non-NULL for the first
	 * time follows...
	 */
}

I.e. dir->exclude_stack is *only* referenced in this function and
dir_clear() (where we also check it for NULL first).

It's state management between calls to prep_exclude(). So that that
initial while-loop can only be entered the the >1th time prep_exclude()
is called.

We'll then either have reached that strbuf_init() already, or if we took
an early return before the strbuf_init() we couldn't have set
dir->exclude_stack either. So that "dir->basebuf.buf" dereference is
safe in either case.