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The strncpy() function is less horrible than strcpy(), but
is still pretty easy to misuse because of its funny
termination semantics. Namely, that if it truncates it omits
the NUL terminator, and you must remember to add it
yourself. Even if you use it correctly, it's sometimes hard
for a reader to verify this without hunting through the
code. If you're thinking about using it, consider instead:
- strlcpy() if you really just need a truncated but
NUL-terminated string (we provide a compat version, so
it's always available)
- xsnprintf() if you're sure that what you're copying
should fit
- strbuf or xstrfmt() if you need to handle
arbitrary-length heap-allocated strings
Note that there is one instance of strncpy in
compat/regex/regcomp.c, which is fine (it allocates a
sufficiently large string before copying). But this doesn't
trigger the ban-list even when compiling with NO_REGEX=1,
because:
1. we don't use git-compat-util.h when compiling it
(instead we rely on the system includes from the
upstream library); and
2. It's in an "#ifdef DEBUG" block
Since it's doesn't trigger the banned.h code, we're better
off leaving it to keep our divergence from upstream minimal.
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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The sprintf() function (and its variadic form vsprintf) make
it easy to accidentally introduce a buffer overflow. If
you're thinking of using them, you're better off either
using a dynamic string (strbuf or xstrfmt), or xsnprintf if
you really know that you won't overflow. The last sprintf()
call went away quite a while ago in f0766bf94e (fsck: use
for_each_loose_file_in_objdir, 2015-09-24).
Note that we respect HAVE_VARIADIC_MACROS here, which some
ancient platforms lack. As a fallback, we can just "guess"
that the caller will provide 3 arguments. If they do, then
the macro will work as usual. If not, then they'll get a
slightly less useful error, like:
git.c:718:24: error: macro "sprintf" passed 3 arguments, but takes just 2
That's not ideal, but it at least alerts them to the problem
area. And anyway, we're primarily targeting people adding
new code. Most developers should be on modern enough
platforms to see the normal "good" error message.
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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The strcat() function has all of the same overflow problems
as strcpy(). And as a bonus, it's easy to end up
accidentally quadratic, as each subsequent call has to walk
through the existing string.
The last strcat() call went away in f063d38b80 (daemon: use
cld->env_array when re-spawning, 2015-09-24). In general,
strcat() can be replaced either with a dynamic string
(strbuf or xstrfmt), or with xsnprintf if you know the
length is bounded.
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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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>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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Jeff King