2bdbe4a2c3
4 Commits
| Author | SHA1 | Message | Date | |
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Jeff King
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e488b7aba7 |
banned.h: mark strncpy() as banned
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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Jeff King
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cc8fdaee1e |
banned.h: mark sprintf() as banned
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
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Jeff King
|
1b11b64b81 |
banned.h: mark strcat() as banned
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
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Jeff King
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c8af66ab8a |
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
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