false positive: `Argument to free() is the address of ..., which is not memory allocated by malloc()`

[phlax]

we are seeing the above error in our CI (running clang-tidy on the https://github.com/envoyproxy/envoy project)

it would appear this is a false positive that should not trigger on a member function

full log:

/b/f/w/source/common/stats/symbol_table.cc:623:3: error: Argument to free() is the address of a local stack variable, which is not memory allocated by malloc() [clang-analyzer-unix.Malloc,-warnings-as-errors]
  table.free(statName());
  ^
/b/f/w/source/common/stats/symbol_table.cc:629:5: note: Calling 'StatNameStorage::free'
    storage.free(symbol_table_);
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~
/b/f/w/source/common/stats/symbol_table.cc:623:3: note: Argument to free() is the address of a local stack variable, which is not memory allocated by malloc()
  table.free(statName());
  ^          ~~~~~~~~~~
/b/f/w/source/common/stats/symbol_table.cc:736:5: error: Argument to free() is the address of the parameter 'stat_name', which is not memory allocated by malloc() [clang-analyzer-unix.Malloc,-warnings-as-errors]
    symbol_table.free(stat_name);
    ^
/b/f/w/source/common/stats/symbol_table.cc:735:3: note: Calling 'StatNameList::iterate'
  iterate([&symbol_table](StatName stat_name) -> bool {
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/b/f/w/source/common/stats/symbol_table.cc:725:24: note: Assuming 'i' is < 'num_elements'
  for (uint32_t i = 0; i < num_elements; ++i) {
                       ^~~~~~~~~~~~~~~~
/b/f/w/source/common/stats/symbol_table.cc:725:3: note: Loop condition is true.  Entering loop body
  for (uint32_t i = 0; i < num_elements; ++i) {
  ^
             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/b/f/w/source/common/stats/symbol_table.cc:736:5: note: Argument to free() is the address of the parameter 'stat_name', which is not memory allocated by malloc()
    symbol_table.free(stat_name);
    ^                 ~~~~~~~~~

[phlax]

cc @ravenblackx

[chrchr-github]

Reduced:

struct S {};
struct T {
	void free(const S& s);
};
void f(T& t) {
	S s;
	t.free(s);
}

<source>:7:2: warning: Argument to free() is the address of the local variable 's', which is not memory allocated by malloc() [clang-analyzer-unix.Malloc]
    7 |         t.free(s);
      |         ^      ~
<source>:7:2: note: Argument to free() is the address of the local variable 's', which is not memory allocated by malloc()
    7 |         t.free(s);
      |         ^      ~

https://godbolt.org/z/qvWrEG3a6

[phlax]

for ref:

$ bazel run //tools/clang-tidy -- --version
...
LLVM (http://llvm.org/):
  LLVM version 14.0.6

[llvmbot]

@llvm/issue-subscribers-clang-static-analyzer

Author: None (phlax)

we are seeing the above error in our CI (running clang-tidy on the https://github.com/envoyproxy/envoy project)

it would appear this is a false positive that should not trigger on a member function

full log:

/b/f/w/source/common/stats/symbol_table.cc:623:3: error: Argument to free() is the address of a local stack variable, which is not memory allocated by malloc() [clang-analyzer-unix.Malloc,-warnings-as-errors]
  table.free(statName());
  ^
/b/f/w/source/common/stats/symbol_table.cc:629:5: note: Calling 'StatNameStorage::free'
    storage.free(symbol_table_);
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~
/b/f/w/source/common/stats/symbol_table.cc:623:3: note: Argument to free() is the address of a local stack variable, which is not memory allocated by malloc()
  table.free(statName());
  ^          ~~~~~~~~~~
/b/f/w/source/common/stats/symbol_table.cc:736:5: error: Argument to free() is the address of the parameter 'stat_name', which is not memory allocated by malloc() [clang-analyzer-unix.Malloc,-warnings-as-errors]
    symbol_table.free(stat_name);
    ^
/b/f/w/source/common/stats/symbol_table.cc:735:3: note: Calling 'StatNameList::iterate'
  iterate([&symbol_table](StatName stat_name) -> bool {
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/b/f/w/source/common/stats/symbol_table.cc:725:24: note: Assuming 'i' is < 'num_elements'
  for (uint32_t i = 0; i < num_elements; ++i) {
                       ^~~~~~~~~~~~~~~~
/b/f/w/source/common/stats/symbol_table.cc:725:3: note: Loop condition is true.  Entering loop body
  for (uint32_t i = 0; i < num_elements; ++i) {
  ^
             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/b/f/w/source/common/stats/symbol_table.cc:736:5: note: Argument to free() is the address of the parameter 'stat_name', which is not memory allocated by malloc()
    symbol_table.free(stat_name);
    ^                 ~~~~~~~~~

[NagyDonat]

The reproducer also triggers the bug on recent clang main; I'll try to investigate and fix the bug.

[NagyDonat]

This kind of confusion between well-known functions and methods that accidentally have the same name probably affects many other checkers as well, because the method bool CallDescription::matches(const CallEvent &) doesn't distinguish function calls and method calls.

I'll try to survey all the 30 checkers that use CallDescriptions and design a solution that would eliminate all occurrences of this issue.

[steakhal]

because the method bool CallDescription::matches(const CallEvent &) doesn't distinguish function calls and method calls.

That was on purpose, and exploited in the llvm-convections checker I think. I agree that this shouldn't be the default behavior. CallDescriptions should be strict about what they match, and be explicit about when you want a fuzzy match.

[NagyDonat]

Yes, I see that there are several checkers (the one checking the getAs() and castAs() methods, and also ContainterModeling.cpp, and probably others as well) that use CallDescriptions to match methods.

I will pay attention to preserve their current behavior while I'm fixing all the other checkers. My rough plan is that by default call descriptions only match functions, and it's possible to specify "is a method call" (and perhaps "may be a method call or a function call") as a flag that's passed to their constructor; but this may evolve as I survey all applications of CallDescriptions.