[static-analyzer] false positive in `unix.Errno`

[tearfur]

The static analyzer considers the case where getcwd() is sucessful but returns nullptr, which shouldn't be possible (if it is, how are we even supposed to check against that??).

Steps:

1. Configure the CMake project.

   CC=clang CXX=clang++ cmake -B build -DCMAKE_BUILD_TYPE=Debug -DCMAKE_CXX_CLANG_TIDY=clang-tidy
   

2. Build the project.

   cd build && cmake --build .
   

Using clang and clang-tidy from the llvm repo.

$ clang --version
Ubuntu clang version 19.0.0 (++20240614073522+cc7a18c18011-1~exp1~20240614193539.212)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/lib/llvm-19/bin
$ clang-tidy --version
Ubuntu LLVM version 19.0.0
  Optimized build.

Code for reproducing

// main.cpp
#include <climits>
#include <iostream>
#include <vector>

#include <unistd.h>

int main()
{
    auto buf = std::vector<char>{};
    buf.resize(PATH_MAX);
    auto path = std::string_view{};
    for (;;)
    {
        if (char const* const ret = getcwd(std::data(buf), std::size(buf)))
        {
            path = ret;
            break;
        }

        if (errno == ERANGE)
        {
            buf.resize(std::size(buf) * 2U);
            continue;
        }
    }

    std::cout << path << '\n';
    return 0;
}

# CMakeLists.txt
cmake_minimum_required(VERSION 3.28)
project(test)

set(CMAKE_CXX_STANDARD 17)

add_executable(test main.cpp)

Output:

/root/main.cpp:20:13: warning: An undefined value may be read from 'errno' [clang-analyzer-unix.Errno]
   20 |         if (errno == ERANGE)
      |             ^
/usr/include/errno.h:38:16: note: expanded from macro 'errno'
   38 | # define errno (*__errno_location ())
      |                ^~~~~~~~~~~~~~~~~~~~~~
/root/main.cpp:12:5: note: Loop condition is true.  Entering loop body
   12 |     for (;;)
      |     ^
/root/main.cpp:14:37: note: Assuming that 'getcwd' is successful; 'errno' becomes undefined after the call
   14 |         if (char const* const ret = getcwd(std::data(buf), std::size(buf)))
      |                                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/root/main.cpp:14:31: note: Assuming 'ret' is null
   14 |         if (char const* const ret = getcwd(std::data(buf), std::size(buf)))
      |                               ^~~
/root/main.cpp:14:9: note: Taking false branch
   14 |         if (char const* const ret = getcwd(std::data(buf), std::size(buf)))
      |         ^
/root/main.cpp:20:13: note: An undefined value may be read from 'errno'
   20 |         if (errno == ERANGE)
      |             ^
/usr/include/errno.h:38:16: note: expanded from macro 'errno'
   38 | # define errno (*__errno_location ())
      |                ^~~~~~~~~~~~~~~~~~~~~~

[llvmbot]

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

Author: Yat Ho (tearfur)

The static analyzer considers the case where `getcwd()` is sucessful but returns `nullptr`, which shouldn't be possible (if it is, how are we even supposed to check against that??).

Steps:

1. Configure the CMake project.

   CC=clang CXX=clang++ cmake -B build -DCMAKE_BUILD_TYPE=Debug -DCMAKE_CXX_CLANG_TIDY=clang-tidy
   

2. Build the project.

   cd build && cmake --build .
   

Using clang and clang-tidy from the llvm repo.

$ clang --version
Ubuntu clang version 19.0.0 (++20240614073522+cc7a18c18011-1~exp1~20240614193539.212)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/lib/llvm-19/bin
$ clang-tidy --version
Ubuntu LLVM version 19.0.0
  Optimized build.

<details>
<summary>Code for reproducing</summary>

// main.cpp
#include <climits>
#include <iostream>
#include <vector>

#include <unistd.h>

int main()
{
    auto buf = std::vector<char>{};
    buf.resize(PATH_MAX);
    auto path = std::string_view{};
    for (;;)
    {
        if (char const* const ret = getcwd(std::data(buf), std::size(buf)))
        {
            path = ret;
            break;
        }

        if (errno == ERANGE)
        {
            buf.resize(std::size(buf) * 2U);
            continue;
        }
    }

    std::cout << path << '\n';
    return 0;
}

# CMakeLists.txt
cmake_minimum_required(VERSION 3.28)
project(test)

set(CMAKE_CXX_STANDARD 17)

add_executable(test main.cpp)

</details>

Output:

/root/main.cpp:20:13: warning: An undefined value may be read from 'errno' [clang-analyzer-unix.Errno]
   20 |         if (errno == ERANGE)
      |             ^
/usr/include/errno.h:38:16: note: expanded from macro 'errno'
   38 | # define errno (*__errno_location ())
      |                ^~~~~~~~~~~~~~~~~~~~~~
/root/main.cpp:12:5: note: Loop condition is true.  Entering loop body
   12 |     for (;;)
      |     ^
/root/main.cpp:14:37: note: Assuming that 'getcwd' is successful; 'errno' becomes undefined after the call
   14 |         if (char const* const ret = getcwd(std::data(buf), std::size(buf)))
      |                                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/root/main.cpp:14:31: note: Assuming 'ret' is null
   14 |         if (char const* const ret = getcwd(std::data(buf), std::size(buf)))
      |                               ^~~
/root/main.cpp:14:9: note: Taking false branch
   14 |         if (char const* const ret = getcwd(std::data(buf), std::size(buf)))
      |         ^
/root/main.cpp:20:13: note: An undefined value may be read from 'errno'
   20 |         if (errno == ERANGE)
      |             ^
/usr/include/errno.h:38:16: note: expanded from macro 'errno'
   38 | # define errno (*__errno_location ())
      |                ^~~~~~~~~~~~~~~~~~~~~~