On Sat Jun 18, 2022 at 12:13 AM CEST, David Malcolm wrote: > 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.
I've now moved the checker inside the region_model. While I got everything working again and fixed the bogus struct error, I'm stuck at the warning if the cast is on a return. Lets take the example: void *create_buffer(int n) { return malloc(n); } int main (int argc, char **argv) { int *buf = create_buffer(42); free (buf); return 0; } After moving it to the region_model, it reaches ctxt->warn (new dubious_allocation_size(...)) but does not emit a warning because inside impl_region_model_context::warn m_stmt and m_stmt_finder are both NULL. m_stmt is null, because the exploded node at which set_value is called on, is the after node of create_buffer (the last bb of the function), which doesn't have any statements. I·tried·to·add·the·call·site·to·the·context·inside program_state::on_edge to·get·the·warning.·While·the·warning·appears, the·notes·in·the·diagnostic are·not·correct·anymore,·i.e.·the·call·site has·the·same·indentation·as the·callee·and·the·return·note·is missing. On the other hand, I can not split the return value set_value call out of pop_frame, because then I'm unable to retrieve the <return_value> as it is already gone. If I simply delay the pop_frame call until the 'before' supernode, I get the warning I wanted but break around 600 test, which can't be right either - at least not for a simple fix. Because I'm stuck at this for some hours, I'd like to ask you what do you think is the best way to get a warning at the call site? - Tim > > > > > > > > >e* attr-malloc-6.c and pr96639.c did both contain stn;cts withn;t 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" > } > | ^~~~~~~~~~~~ For anyone in the future with the same problem. Make sure to check that TYPE_SIZE_UNIT != NULL_TREE before calling size_in_bytes. > > 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