Subject: Re: [PATCH v5 1/3] difftool: fix symlink-file writing in dir-diff mode

From: Junio C Hamano <[email-masked]>
Date: Thu, 23 Sep 2021 14:46:29 -0700

David Aguilar <[email-masked]> writes:

> diff --git a/builtin/difftool.c b/builtin/difftool.c
> index bb9fe7245a..21e055d13a 100644
> --- a/builtin/difftool.c
> +++ b/builtin/difftool.c
> @@ -557,11 +557,13 @@ static int run_dir_diff(const char *extcmd, int symlinks, const char *prefix,
>  		if (*entry->left) {
>  			add_path(&ldir, ldir_len, entry->path);
>  			ensure_leading_directories(ldir.buf);
> +			unlink(ldir.buf);
>  			write_file(ldir.buf, "%s", entry->left);
>  		}
>  		if (*entry->right) {
>  			add_path(&rdir, rdir_len, entry->path);
>  			ensure_leading_directories(rdir.buf);
> +			unlink(rdir.buf);
>  			write_file(rdir.buf, "%s", entry->right);
>  		}
>  	}

Curiously, this pattern repeats twice in the vicinity of the code.
We cannot see it because it is out of pre-context, but the above is
a body of a loop that iterates over "symlinks2" hashmap.  There is
another identical loop that iterates over "submodules", and we are
not protecting ourselves from following a stray/leftover symbolic
link in the loop.

I wonder if we should do the same to be defensive?  I also wondered
if write_file() should be the one that may want to be doing the
unlink(), but I ran out of time before I finished reading all the
callers to see if that is even a correct thing to do (meaning: some
caller may want to truly overwrite an existing file, and follow
symlinks if there already is, and I didn't audit all callers to see
if there is no such caller).

The two identical looking loops also look like an accident waiting
to happen---a patch like this that wants to touch only one of them
would risk application to the other, wrong, loop if the patch gets
old enough and patch offset grows larger ;-).

Thanks.