[PATCH] [StaticAnalyzer] New checker Sizeof on expression

Anders Rönnholm Anders.Ronnholm at evidente.se
Mon Nov 25 01:29:33 PST 2013


New iteration on this checker I'd like to get reviewed. I have moved the checker to Sema and avoid SFINAE.


From: Anna Zaks [mailto:ganna at apple.com]
Sent: den 18 november 2013 18:39
To: Anders Rönnholm
Cc: Richard Smith; Ted Kremenek; cfe-commits at cs.uiuc.edu; Jordan Rose
Subject: Re: [PATCH] [StaticAnalyzer] New checker Sizeof on expression

On Nov 18, 2013, at 1:56 AM, Anders Rönnholm <Anders.Ronnholm at evidente.se<mailto:Anders.Ronnholm at evidente.se>> wrote:


I have run the checker on llvm codebase and chrome and did not find any false positives. Not any true positives either. I also ran it on some SFINAE examples with no false positives. But that was expected as I have not seen any SFINAE examples where they use a binary expression in the sizeof nor sizeof(sizeof()), is that really common?

I saw that there is an isSFINAEContext function in semaexpr. What do you think about moving the check to sema and use that function to avoid triggering on SFINAE?

Adding the test case referenced by Richard, moving the check to Sema, and resubmitting the patch sound like a good plan to me.



From: Anna Zaks [mailto:ganna at apple.com]
Sent: den 23 oktober 2013 17:35
To: Daniel Marjamäki
Cc: Ted Kremenek; Anders Rönnholm; Richard Smith; cfe-commits at cs.uiuc.edu<mailto:cfe-commits at cs.uiuc.edu>; Jordan Rose
Subject: Re: [PATCH] [StaticAnalyzer] New checker Sizeof on expression


I think Richard was asking about false positives/true positives ratio and I do not see any false positives in the list below. It looks like all the reports you found are valid. Did you run this check on large C++ codebases?

As far as I can tell there are two outstanding issues:
 - Addressing these questions from Richard:
sizeof(expression) is a common idiom in SFINAE contexts. Is that covered here?
Richard, can you provide examples?

sizeof(sizeof(int)) is a "traditional" way to get sizeof(size_t). Why should we warn on that?
(I personally think that if this check goes into the analyzer, it's fine to warn in the second case.)

 - Where this check should go (compiler or the analyzer)?
I think that if you can address the questions above, this check could go into the compiler.


On Oct 14, 2013, at 12:43 PM, Daniel Marjamäki <Daniel.Marjamaki at evidente.se<mailto:Daniel.Marjamaki at evidente.se>> wrote:


As I understood it.. you wanted to know what the hit ratio is. I have investigated the hit ratio on a number of packages in debian. Below is the negative and positives I've found. I provide the package URLs so you can look for yourself..

I will continue investigating and let you know if I see any false positives...

NEGATIVE (macro)
package: http://ftp.us.debian.org/debian/pool/main/a/agedu/agedu_9723.orig.tar.gz

#define sresize(array, number, type) \
    ( (void)sizeof((array)-(type *)0), \
      (type *) srealloc ((array), (number) * sizeof (type)) )

We don't warn about this since it's in a macro. However it is interesting:
    sizeof((array)-(type *)0)
It seems to be written this way by intention. To get sizeof(ptrdiff_t)? Maybe sizeof(ptr-ptr) should be ok.

package: http://ftp.us.debian.org/debian/pool/main/a/argus-client_3.0.0.orig.tar.gz

file: argus-clients-3.0.0/radump/print-ldp.c
line: 607
            print_unknown_data(tptr+sizeof(sizeof(struct ldp_msg_header)),"\n\t  ",

file: argus-clients-3.0.0/radump/print-lmp.c
line: 878
            print_unknown_data(tptr+sizeof(sizeof(struct lmp_object_header)),"\n\t    ",
                               lmp_obj_len-sizeof(struct lmp_object_header));

Not sure if these are intended to be sizeof(size_t) or sizeof(struct lmp_object_header) but I guess the latter.

package: http://ftp.us.debian.org/debian/pool/main/b/babel/babel_1.4.0.dfsg.orig.tar.gz
file: babel-1.4.0.dfsg/regression/output/libC/synch_RegOut_Impl.c

332:    if ((res < 0) || (res > sizeof(s_result_strings)/sizeof(sizeof(char *)))) {

This "sizeof(sizeof(char *))" should be "sizeof(char*)" since s_result_strings is a array of char * pointers

package: http://ftp.us.debian.org/debian/pool/main/b/bird/bird_1.3.7.orig.tar.gz
file: bird-1.3.7/lib/mempool.c

  return ALLOC_OVERHEAD + sizeof(struct linpool) +
    cnt * (ALLOC_OVERHEAD + sizeof(sizeof(struct lp_chunk))) +
    m->total + m->total_large;

Not sure if this is intended to be sizeof(size_t) or sizeof(struct lp_chunk) but I guess the latter.

package: http://ftp.us.debian.org/debian/pool/main/b/blender/blender_2.49.2~dfsg.orig.tar.gz
file: blender-2.49b.orig/source/blender/blenloader/intern/readfile.c
6987:    if(0==strncmp(ima->id.name+2, "Viewer Node", sizeof(ima->id.name+2))) {
6991:    if(0==strncmp(ima->id.name+2, "Render Result", sizeof(ima->id.name+2))) {

These are bad, ima->id.name is an array, it should probably be changed to:
    if(0==strncmp(ima->id.name+2, "Viewer Node", sizeof(ima->id.name) - 2)) {

Best regards,
Daniel Marjamäki

Daniel Marjamäki Senior Engineer
Evidente ES East AB  Warfvinges väg 34  SE-112 51 Stockholm  Sweden

Mobile:                 +46 (0)709 12 42 62
E-mail:                 Daniel.Marjamaki<mailto:Daniel.Marjamaki at evidente.se>@evidente.se<mailto:Daniel.Marjamaki at evidente.se>

Från: cfe-commits-bounces at cs.uiuc.edu<mailto:cfe-commits-bounces at cs.uiuc.edu> [cfe-commits-bounces at cs.uiuc.edu<mailto:cfe-commits-bounces at cs.uiuc.edu>] för Anna Zaks [ganna at apple.com<mailto:ganna at apple.com>]
Skickat: den 8 oktober 2013 18:18
Till: Ted Kremenek
Cc: Anders Rönnholm; Richard Smith; cfe-commits at cs.uiuc.edu<mailto:cfe-commits at cs.uiuc.edu>
Ämne: Re: [PATCH] [StaticAnalyzer] New checker Sizeof on expression

On Oct 7, 2013, at 10:05 PM, Ted Kremenek <kremenek at apple.com<mailto:kremenek at apple.com>> wrote:

On Oct 7, 2013, at 6:05 PM, Jordan Rose <jordan_rose at apple.com<mailto:jordan_rose at apple.com>> wrote:

On Oct 7, 2013, at 13:58, Richard Smith <richard at metafoo.co.uk<mailto:richard at metafoo.co.uk>> wrote:

On Mon, Oct 7, 2013 at 9:55 AM, Jordan Rose <jordan_rose at apple.com<mailto:jordan_rose at apple.com>> wrote:
I'm fine with this staying in the analyzer for now, unless David, Richard, or Eli feel it should be a warning right away.

Do we have evidence that we want this? Does it catch bugs? If so, what do they look like? It seems like this would trigger on legitimate code; how does a user suppress the warning in that case, and does that suppression make their code clearer?

What is the false/true positive ratio for bug finding here?

sizeof(expression) is a common idiom in SFINAE contexts. Is that covered here?

sizeof(sizeof(int)) is a "traditional" way to get sizeof(size_t). Why should we warn on that?

And more as a general question than something specific to this patch: Is there a region in the space of false positive ratios where we think a syntactic warning should go into the static analyzer? If so, why? And what is that region? I would have thought that the static analyzer, like the clang warnings, would be aimed at (eventually) having a false positive ratio of near zero. If so, then should we ever put a warning in the static analyzer if it doesn't require the static analyzer's technology (or have a high runtime cost)?

On this last (and bringing in Ted and Anna):

I think the main difference between compiler warnings and syntactic analyzer checks is that we try very hard to turn new compiler warnings on by default. A second-order effect of this is that we generally avoid style warnings. The analyzer can be a bit looser about this, though: because people know the analyzer is stricter and more in-depth, I think they might also accept that a particular check doesn't fit their project.

On the other hand, we still haven't gotten around to designing a proper bug tracking and/or manual suppression system, so that's one advantage of compiler warnings. And as you say, checks without a high runtime cost don't really have a technical reason to be in the analyzer.

Richard's point is correct that we want the static analyzer to also have a high signal-to-noise ratio.  Otherwise it is a useless tool.  I'm also not a fan of having the analyzer having a bunch of "junk" checkers that aren't on by default, but if a checker, when it is enabled, is HIGHLY useful to some set of people (e.g., security experts who are more tolerant of false positive rates if they want to do an aggressive code audit)  I think the analyzer is a reasonable place to put them, given the current warning policy in Clang where we want the warnings there to be generally useful to everybody.

To put it in more context, in the beginning the guiding principal of what goes in the static analyzer was:

(a) The warning is very expensive to compute.


(b) The warning is very domain-specific.  For example, an API such as CFNumberCreate() on OS X has some interesting API invariants, but we generally should not be hacking those API-specific warnings into the compiler.  Exceptions exist of course, e.g., printf checking, but often the are grounded when such APIs are fairly standard (e.g., in the C standard itself) or the checking is based on some annotation like __attribute__((format)) where the compiler doesn't know anything about a specific function itself, just the annotation.

Style warnings can sometimes fall into (a) (in which case not putting them in the compiler makes obvious sense), but one could argue that they are more the flavor of (b) then a traditional compiler warning.  As I mentioned earlier, the static analyzer can be home to some highly specialized checkers that may not be generally useful for everybody but when enabled are very useful to certain people.  Style warnings often fit in this category of warnings.

A related problem is that we don't have an ontology for style warnings in clang (the compiler).  Should they really be lumped into the same group of all other compiler warnings?  What about their behavior with -Werror?  -Weverything?  Style warnings really are about personal style, and users can be highly polarized about them.  For example, it would be highly useful for Clang to have a -W80-columns or -Wspaces-instead-of-tabs (not a serious proposal).  Both would be cheap to compute, would obviously benefit LLVM developers, but they wouldn't work for everybody, or even the majority of software developers.  Where should they go?  The compiler?  The static analyzer?  Both have discoverability and usability concerns in either location.  I'd argue that these two warnings would be better suited in the compiler because (when the user wants them) they'd get run all the time, but having these warnings counteracts the general guiding principal that compiler warnings should generally be useful to most people, or clear categories of people (e.g, library authors), but not specific groups of people (e.g., LLVM developers).

This is not a thought out proposal, but I would not be opposed to a new category of warnings, say -Wstyle, and have all style warnings under -Wstyle, perhaps named "-Wstyle-80-columns" or "-Wstyle-spaces-instead-of-tabs".  None of these warnings would be on by default.  If we want these kind of warnings in the compiler, I think these kind of warnings are worth calling out as being something "different" that people shouldn't get hung up about if they don't think the warning applies to them.  Also, having them in a special category of them own allows institutions to set broad warning polices such as "-Werror -Wno-error=style" where they want regular warnings to be treated as error, but not style warnings, etc.

I agree with Ted. Currently, there is no good way of adding style and highly specialized warnings to clang; given the behavior of Werror and Weverything. For example, we encourage users to learn about new generic warnings by turning on Weverything, so the specialized warnings should not be included there.

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