See e.g. bug 80049, where we have code of the form expr = p->field; [...] if (p) ... We could complain that 'p' has already been dereferenced before the check. This might be implementable using sm-malloc: the "p->field" dereference transitions "p" to non-null, and we could thus complain at the "if (p)" on non-null if the transition was due to such a transition (rather than being known to be non-null) - or perhaps a new state "assumed-non-null"? PVS Studio complains about this: https://www.viva64.com/en/w/v595/ https://www.viva64.com/en/examples/v595/ using CWE 476: https://cwe.mitre.org/data/definitions/476.html
I have a mostly-working -fanalyzer implementation of this. I now see that PR 77432 tracks implementing this, albeit for the middle-end.
Created attachment 53863 [details] Implementation of this (not yet ported to Sphinx) This patch implements the new warning; still uses texinfo rather than sphinx. Not yet posted to mailing list; am waiting on dust to settle from the texinfo->sphinx transition on trunk.
The master branch has been updated by David Malcolm <dmalcolm@gcc.gnu.org>: https://gcc.gnu.org/g:5c6546ca7d8cab1f1c129f5b55f709e2ceee0f94 commit r13-3884-g5c6546ca7d8cab1f1c129f5b55f709e2ceee0f94 Author: David Malcolm <dmalcolm@redhat.com> Date: Thu Nov 10 13:23:56 2022 -0500 analyzer: new warning: -Wanalyzer-deref-before-check [PR99671] This patch implements a new -Wanalyzer-deref-before-check within -fanalyzer. It complains about code paths in which a pointer is checked for NULL after it has already been dereferenced. For example, for the testcase in PR 77432 the diagnostic emits: deref-before-check-1.c: In function 'test_from_pr77432': deref-before-check-1.c:6:8: warning: check of 'a' for NULL after already dereferencing it [-Wanalyzer-deref-before-check] 6 | if (a) | ^ 'test_from_pr77432': events 1-2 | | 5 | int b = *a; | | ^ | | | | | (1) pointer 'a' is dereferenced here | 6 | if (a) | | ~ | | | | | (2) pointer 'a' is checked for NULL here but it was already dereferenced at (1) | and in PR 77425 we had an instance of this hidden behind a macro, which the diagnostic complains about as follows: deref-before-check-pr77425.c: In function 'get_odr_type': deref-before-check-pr77425.c:35:10: warning: check of 'odr_types_ptr' for NULL after already dereferencing it [-Wanalyzer-deref-before-check] 35 | if (odr_types_ptr) | ^ 'get_odr_type': events 1-3 | | 27 | if (cond) | | ^ | | | | | (1) following 'false' branch... |...... | 31 | else if (other_cond) | | ~~~~~~~~~~~ | | || | | |(2) ...to here | | (3) following 'true' branch... | 'get_odr_type': event 4 | | 11 | #define odr_types (*odr_types_ptr) | | ~^~~~~~~~~~~~~~~ | | | | | (4) ...to here deref-before-check-pr77425.c:33:7: note: in expansion of macro 'odr_types' | 33 | odr_types[val->id] = 0; | | ^~~~~~~~~ | 'get_odr_type': event 5 | | 11 | #define odr_types (*odr_types_ptr) | | ~^~~~~~~~~~~~~~~ | | | | | (5) pointer 'odr_types_ptr' is dereferenced here deref-before-check-pr77425.c:33:7: note: in expansion of macro 'odr_types' | 33 | odr_types[val->id] = 0; | | ^~~~~~~~~ | 'get_odr_type': event 6 | | 35 | if (odr_types_ptr) | | ^ | | | | | (6) pointer 'odr_types_ptr' is checked for NULL here but it was already dereferenced at (5) | gcc/analyzer/ChangeLog: PR analyzer/99671 * analyzer.opt (Wanalyzer-deref-before-check): New warning. * diagnostic-manager.cc (null_assignment_sm_context::set_next_state): Only add state change events for transition to "null" state. (null_assignment_sm_context::is_transition_to_null): New. * engine.cc (impl_region_model_context::on_pop_frame): New. * exploded-graph.h (impl_region_model_context::on_pop_frame): New decl. * program-state.cc (sm_state_map::clear_any_state): New. (sm_state_map::can_merge_with_p): New. (program_state::can_merge_with_p): Replace requirement that sm-states be equal in favor of an attempt to merge them. * program-state.h (sm_state_map::clear_any_state): New decl. (sm_state_map::can_merge_with_p): New decl. * region-model.cc (region_model::eval_condition): Make const. (region_model::pop_frame): Call ctxt->on_pop_frame. * region-model.h (region_model::eval_condition): Make const. (region_model_context::on_pop_frame): New vfunc. (noop_region_model_context::on_pop_frame): New. (region_model_context_decorator::on_pop_frame): New. * sm-malloc.cc (enum resource_state): Add RS_ASSUMED_NON_NULL. (allocation_state::dump_to_pp): Drop "final". (struct assumed_non_null_state): New subclass. (malloc_state_machine::m_assumed_non_null): New. (assumed_non_null_p): New. (class deref_before_check): New. (assumed_non_null_state::dump_to_pp): New. (malloc_state_machine::get_or_create_assumed_non_null_state_for_frame): New. (malloc_state_machine::maybe_assume_non_null): New. (malloc_state_machine::on_stmt): Transition from start state to "assumed-non-null" state for pointers passed to __attribute__((nonnull)) arguments, and for pointers explicitly dereferenced. Call maybe_complain_about_deref_before_check for pointers explicitly compared against NULL. (malloc_state_machine::maybe_complain_about_deref_before_check): New. (malloc_state_machine::on_deallocator_call): Also transition "assumed-non-null" states to "freed". (malloc_state_machine::on_pop_frame): New. (malloc_state_machine::maybe_get_merged_states_nonequal): New. * sm-malloc.dot: Update for changes to sm-malloc.cc. * sm.h (state_machine::on_pop_frame): New. (state_machine::maybe_get_merged_state): New. (state_machine::maybe_get_merged_states_nonequal): New. gcc/ChangeLog: * doc/gcc/gcc-command-options/options-that-control-static-analysis.rst: Add -Wanalyzer-deref-before-check. gcc/testsuite/ChangeLog: * gcc.dg/analyzer/deref-before-check-1.c: New test. * gcc.dg/analyzer/deref-before-check-2.c: New test. * gcc.dg/analyzer/deref-before-check-pr77425.c: New test. * gcc.dg/analyzer/malloc-1.c (test_51): New test. gcc/ChangeLog: PR analyzer/99671 * tristate.h (tristate::is_unknown): New. Signed-off-by: David Malcolm <dmalcolm@redhat.com>
Implemented for GCC 13 by the above patch; closing this RFE.