The rest apply cleanly (because they're far enough away textually from the definition of BANNED).
So here's a replacement for just patch 1 (I'm assuming this creates less work than re-posting them all, but it may not be if Junio prefers dealing with a whole new mbox rather than a "rebase -i", "reset --hard HEAD^", "git am" -- let me know if you'd prefer it the other way).
-- >8 --
Subject: [PATCH] automatically ban strcpy()
There are a few standard C functions (like strcpy) which are easy to misuse. E.g.:
char path[PATH_MAX];
strcpy(path, arg);
may overflow the "path" buffer. Sometimes there's an earlier constraint on the size of "arg", but even in such a case it's hard to verify that the code is correct. If the size really is unbounded, you're better off using a dynamic helper like strbuf:
struct strbuf path = STRBUF_INIT;
strbuf_addstr(path, arg);
or if it really is bounded, then use xsnprintf to show your expectation (and get a run-time assertion):
char path[PATH_MAX];
xsnprintf(path, sizeof(path), "%s", arg);
which makes further auditing easier.
We'd usually catch undesirable code like this in a review, but there's no automated enforcement. Adding that enforcement can help us be more consistent and save effort (and a round-trip) during review.
This patch teaches the compiler to report an error when it sees strcpy (and will become a model for banning a few other functions). This has a few advantages over a separate linting tool:
1. We know it's run as part of a build cycle, so it's
hard to ignore. Whereas an external linter is an extra
step the developer needs to remember to do. 2. Likewise, it's basically free since the compiler is
parsing the code anyway. 3. We know it's robust against false positives (unlike a
grep-based linter).The two big disadvantages are:
1. We'll only check code that is actually compiled, so it
may miss code that isn't triggered on your particular
system. But since presumably people don't add new code
without compiling it (and if they do, the banned
function list is the least of their worries), we really
only care about failing to clean up old code when
adding new functions to the list. And that's easy
enough to address with a manual audit when adding a new
function (which is what I did for the functions here). 2. If this ends up generating false positives, it's going
to be harder to disable (as opposed to a separate
linter, which may have mechanisms for overriding a
particular case). But the intent is to only ban functions which are
obviously bad, and for which we accept using an
alternative even when this particular use isn't buggy
(e.g., the xsnprintf alternative above).The implementation here is simple: we'll define a macro for the banned function which replaces it with a reference to a descriptively named but undeclared identifier. Replacing it with any invalid code would work (since we just want to break compilation). But ideally we'd meet these goals:
- it should be portable; ideally this would trigger
everywhere, and does not need to be part of a DEVELOPER=1
setup (because unlike warnings which may depend on the
compiler or system, this is a clear indicator of
something wrong in the code).
- it should generate a readable error that gives the
developer a clue what happened
- it should avoid generating too much other cruft that
makes it hard to see the actual error
- it should mention the original callsite in the error
The output with this patch looks like this (using gcc 7, on a checkout with 022d2ac1f3 reverted, which removed the final strcpy from blame.c):
CC builtin/blame.o
In file included from ./git-compat-util.h:1246,
from ./cache.h:4,
from builtin/blame.c:8:
builtin/blame.c: In function ‘cmd_blame’:
./banned.h:11:22: error: ‘sorry_strcpy_is_a_banned_function’ undeclared (first use in this function)
#define BANNED(func) sorry_##func##_is_a_banned_function
^~~~~~
./banned.h:14:21: note: in expansion of macro ‘BANNED’
#define strcpy(x,y) BANNED(strcpy)
^~~~~~
builtin/blame.c:1074:4: note: in expansion of macro ‘strcpy’
strcpy(repeated_meta_color, GIT_COLOR_CYAN);
^~~~~~
./banned.h:11:22: note: each undeclared identifier is reported only once for each function it appears in
#define BANNED(func) sorry_##func##_is_a_banned_function
^~~~~~
./banned.h:14:21: note: in expansion of macro ‘BANNED’
#define strcpy(x,y) BANNED(strcpy)
^~~~~~
builtin/blame.c:1074:4: note: in expansion of macro ‘strcpy’
strcpy(repeated_meta_color, GIT_COLOR_CYAN);
^~~~~~This prominently shows the phrase "strcpy is a banned function", along with the original callsite in blame.c and the location of the ban code in banned.h. Which should be enough to get even a developer seeing this for the first time pointed in the right direction.
This doesn't match our ideals perfectly, but it's a pretty good balance. A few alternatives I tried:
1. Instead of using an undeclared variable, using an
undeclared function. This shortens the message, because
the "each undeclared identifier" message is not needed
(and as you can see above, it triggers a separate
mention of each of the expansion points). But it doesn't actually stop compilation unless you use
-Werror=implicit-function-declaration in your CFLAGS.
This is the case for DEVELOPER=1, but not for a default
build (on the other hand, we'd eventually produce a
link error pointing to the correct source line with the
descriptive name). 2. The linux kernel uses a similar mechanism in its
BUILD_BUG_ON_MSG(), where they actually declare the
function but do so with gcc's error attribute. But
that's not portable to other compilers (and it also
runs afoul of our error() macro). We could make a gcc-specific technique and fallback on
other compilers, but it's probably not worth the
complexity. It also isn't significantly shorter than
the error message shown above. 3. We could drop the BANNED() macro, which would shorten
the number of lines in the error. But curiously,
removing it (and just expanding strcpy directly to the
bogus identifier) causes gcc _not_ to report the
original line of code.So this strategy seems to be an acceptable mix of information, portability, simplicity, and robustness, without _too_ much extra clutter. I also tested it with clang, and it looks as good (actually, slightly less cluttered than with gcc).
Signed-off-by: Jeff King <peff@peff.net>
---
banned.h | 16 ++++++++++++++++
git-compat-util.h | 6 ++++++
2 files changed, 22 insertions(+)
create mode 100644 banned.h
Jeff King
