On Fri, 2022-06-17 at 22:23 +0200, Tim Lange wrote: > On Fri, Jun 17, 2022 at 01:48:09PM -0400, David Malcolm wrote: > > On Fri, 2022-06-17 at 17:54 +0200, Tim Lange wrote:
[...snip...] > > > > I have resent the patch using git send-email as a reply to my original > message. > The new message looks properly formatted in the archive: > https://gcc.gnu.org/pipermail/gcc/2022-June/238911.html Thanks; that's *much* more readable. [...snip...] > > > > > > > > On symbolic buffer sizes: > > warning: Allocated buffer size is not a multiple of the pointee's > > size > > [CWE-131] [-Wanalyzer-allocation-size] > > 33 | int *ptr = malloc (n + sizeof(int)); /* { dg-line malloc3 } > > */ > > | ^~~~~~~~~~~~~~~~~~~~~~~~ > > ‘test_3’: event 1 > > | > > | 33 | int *ptr = malloc (n + sizeof(int)); /* { dg-line malloc3 > > } > > */ > > | | ^~~~~~~~~~~~~~~~~~~~~~~~ > > | | | > > | | (1) Allocation is incompatible with ‘int *’; either the > > allocated size is bogus or the type on the left-hand side is wrong > > | > > > > > > Is there location information for both the malloc and for the > > assignment, here? > > I'm not sure whether I understand your question but the warning is > emitted at the gcall* with a ssa var lhs and the call_fndecl on the > rhs. > I think that is enough to split that up into "(1) n + sizeof(int) > allocated here" and "(2) Allocation at (1) is incompatible with..."? Probably, yes. FWIW I wrote some more notes about the events in my reply to to your reply to Prathamesh, here: https://gcc.gnu.org/pipermail/gcc/2022-June/238917.html [...snip...] > > > > There are some things to discuss from my side: > > * The tests with the "toy re-implementation of CPython's object > > model"[2] fail due to a extra warning emitted. Because the analyzer > > can't know the calculation actually results in a correct buffer size > > when viewed as a string_obj later on, it emits a warning, e.g. at > > line > > 61 in data-model-5.c. The only mitigation would be to disable the > > warning for structs entirely. Now, the question is to rather have > > noise > > on these cases or disable the warning for structs entirely? > > > > Can you post the full warning please? > > /path/to/data-model-5.c: In function ‘alloc_obj’: > /path/to/data-model-5.c:61:31: warning: Allocated buffer size is not a > multiple of the pointee's size [CWE-131] [-Wanalyzer-allocation-size] > 61 | base_obj *obj = (base_obj *)malloc (sz); > | ^~~~~~~~~~~ > ‘new_string_obj’: events 1-2 > | > | 69 | base_obj *new_string_obj (const char *str) > | | ^~~~~~~~~~~~~~ > | | | > | | (1) entry to ‘new_string_obj’ > |...... > | 75 | = (string_obj *)alloc_obj (&str_type, sizeof > (string_obj) + len + 1); > | | > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > | | | > | | (2) calling ‘alloc_obj’ from > ‘new_string_obj’ > | > +--> ‘alloc_obj’: events 3-4 > | > | 59 | base_obj *alloc_obj (type_obj *ob_type, size_t sz) > | | ^~~~~~~~~ > | | | > | | (3) entry to ‘alloc_obj’ > | 60 | { > | 61 | base_obj *obj = (base_obj *)malloc (sz); > | | ~~~~~~~~~~~ > | | | > | | (4) Allocation is > incompatible with ‘base_obj *’; either the allocated size is bogus or > the type on the left-hand side is wrong > | > > > > > These testcases exhibit a common way of faking inheritance in C, and > > I > > think it ought to be possible to support this in the warning. > > > > I thing what's happening is we have > > > > struct base > > { > > /* fields */ > > }; > > > > struct sub > > { > > struct base m_base; > > /* extra fields. */ > > }; > > > > struct base *construct_base (size_t sz) > > { > > struct base *p = (struct base *) malloc (sz); > > > > /* set up fields of base in p */ > > > > return p; > > } > > > > Or is this on the interprocedural path as called with a specific > > sizeof > > for struct sub? > > At (4), it does not know that base_obj is later used as a "base > struct". > As it is called with sizeof(struct sub), my checker thinks the buffer > is > too large for one but too small for another base_obj. > > > > > Maybe we can special-case these by detecting where struct sub's first > > field is struct base, and hence where we expect this pattern? (and > > use > > this to suppress the warning for such cases?) > > I already excluded all structs with structs inside with > struct_or_union_with_inheritance_p inside sm-malloc.cc. This does not > help > in the case size for struct sub is allocated but casted as base. Maybe, > we > should do a special case for structs where we only warn when the sizeof > is > too small to hold the base struct together with supressing warnings > when > the first field is a struct? That sounds like it could work. There are several things going on in the above example: - fake inheritance - the "trailing array idiom": struct string_obj's final field is: char str_buf[]; meaning that the string_obj will have the char buffer trailing off the end, and the allocation is expected to support this. This is not uncommon in C; it occurs in CPython, see e.g.: https://github.com/python/cpython/blob/main/Include/cpython/bytesobject.h where CPython's PyBytesObject has the bytes in an ob_sval field trailing off the end: typedef struct { PyObject_VAR_HEAD Py_DEPRECATED(3.11) Py_hash_t ob_shash; char ob_sval[1]; /* Invariants: * ob_sval contains space for 'ob_size+1' elements. * ob_sval[ob_size] == 0. * ob_shash is the hash of the byte string or -1 if not computed yet. */ } PyBytesObject; so it would be good for the warning to handle it gracefully, which I think your proposal above would. I try to have plenty of idiomatic C code in the analyzer test suite to try to catch this kind of thing (as well as more "unit test" kinds of test coverage); we want the warnings to have a good signal:noise ratio. > > > * I'm unable to emit a warning whenever the cast happens at an > assignment with a call as the rhs, e.g. test_1 in allocation-size-4.c. > This is because I'm unable to access a region_svalue for the returned > value. Even in the new_program_state, the svalue of the lhs is still a > conjured_svalue. Maybe David can lead me to a place where I can access > the return value's region_svalue or do I have to adapt the engine? > > Please can you try reposting the patch? I tried to read it, but am > having trouble with the mangled indentation. See my inline answer above. Both, the test case and from where I want to access the region_svalue are commented with // FIXME. What does the dump of the state look like? e.g. via calling (gdb) call m_region_model->debug() from within gdb A conjured_svalue represents the result of a call to an external function (or a side-effect written out to a *out-style param of such a function), but we have the body of create_buffer, so the call to create_buffer should be analyzed interprocedurally, and we should have a region_svalue pointing at a heap_allocated_region. You might want to simplify things to just the functions of interest, and then have a look at the output of -fdump-analyzer-exploded-graph in your favorite .dot viewer (I like xdot; it's in python-xdot in Fedora). I wonder if my idea from the other email of moving the test from sm- malloc.cc to region-model.cc might affect this; the state machines run at a slightly different time to the region model updates. > > > * attr-malloc-6.c and pr96639.c did both contain structs without an > implementation. Something in the analyzer must have triggered another > warning about the usage of those without them having an implementation. > I changed those structs to have an empty implementation, such that the > additional warning are gone. I think this shouldn't change the test > case, so is this change okay? > > What were the new warnings? /path/to/attr-malloc-6.c:175:15: error: invalid use of undefined type ‘struct FILE’ 175 | FILE *p = malloc (100); // { dg-message "allocated here" } | ^~~~~~~~~~~~ All were like the one above. error: invalid use of undefined type 'struct XXX' That error looks bogus; I'm guessing that something the new diagnostics is calling is generating it. You can probably track it down by using (gdb) break-on-diagnostic in the debugger, and then seeing what the backtrace shows when the breakpoint fires. See: https://gcc-newbies-guide.readthedocs.io/en/latest/debugging.html break-on-diagnostic is one of the things in the support scripts mentioned on that page. Hope this is helpful (BTW, I'm about to disappear for a long weekend; I'm back on Tuesday) Dave