FEXLoader: Drop the binfmt_misc I flag

Upstream has rejected this flag which would have let FEX close the gap
in functionality between binfmt_misc interpreters and PT_INTERP
interpreters for how `/proc/exe` is handled. Since we are unable to
change their opinions, just remove the code from FEX since it's never
going to happen.

Removes a little bit of confusing behaviour in execveat and
filemanagement handling. Leaving us with only the regular confusing
behaviour of trying to track accesses to `/proc/exe` instead.

Data/binfmts/FEX-x86.in

@@ -6,4 +6,3 @@ mask \xff\xff\xff\xff\xff\xfe\xfe\x00\x00\x00\x00\xff\xff\xff\xff\xff\xfe\xff\xf
 credentials yes
 fix_binary yes
 preserve yes
-expose_interpreter optional

Data/binfmts/FEX-x86_64.in

@@ -6,4 +6,3 @@ mask \xff\xff\xff\xff\xff\xfe\xfe\x00\x00\x00\x00\xff\xff\xff\xff\xff\xfe\xff\xf
 credentials yes
 fix_binary yes
 preserve yes
-expose_interpreter optional

Source/Tools/FEXLoader/CMakeLists.txt

@@ -96,17 +96,6 @@ if (CMAKE_SYSTEM_PROCESSOR MATCHES "aarch64")
         add_dependencies(uninstall uninstall_binfmt_misc_64)
       endif()
     else()
-      set (SUPPORTED_BINFMT_MISC_FLAGS "POCF")
-      execute_process(COMMAND uname -r OUTPUT_VARIABLE UNAME_VERSION OUTPUT_STRIP_TRAILING_WHITESPACE)
-      string(REGEX MATCH "[0-9]+.[0-9]+" KERNEL_VERSION ${UNAME_VERSION})
-      message(STATUS "Kernel version: ${KERNEL_VERSION}")
-
-      if (KERNEL_VERSION VERSION_GREATER_EQUAL 9999.0)
-        # New binfmt_misc flag for exposing the interpreter was added in version '9999.0'
-        # Only enable it if the host kernel is at least that.
-        set (SUPPORTED_BINFMT_MISC_FLAGS "POCFI")
-      endif()
-
       # In the case of update-binfmts not being available (Arch for example) then we need to install manually
       add_custom_target(binfmt_misc_32
         COMMAND ${CMAKE_COMMAND} -E
@@ -117,7 +106,7 @@ if (CMAKE_SYSTEM_PROCESSOR MATCHES "aarch64")
           echo "Attempting to install FEX-x86 misc now."
         COMMAND ${CMAKE_COMMAND} -E
           echo
-          ':FEX-x86:M:0:\\x7fELF\\x01\\x01\\x01\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x02\\x00\\x03\\x00:\\xff\\xff\\xff\\xff\\xff\\xfe\\xfe\\x00\\x00\\x00\\x00\\xff\\xff\\xff\\xff\\xff\\xfe\\xff\\xff\\xff:${CMAKE_INSTALL_PREFIX}/bin/FEXInterpreter:${SUPPORTED_BINFMT_MISC_FLAGS}' > /proc/sys/fs/binfmt_misc/register
+          ':FEX-x86:M:0:\\x7fELF\\x01\\x01\\x01\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x02\\x00\\x03\\x00:\\xff\\xff\\xff\\xff\\xff\\xfe\\xfe\\x00\\x00\\x00\\x00\\xff\\xff\\xff\\xff\\xff\\xfe\\xff\\xff\\xff:${CMAKE_INSTALL_PREFIX}/bin/FEXInterpreter:POCF' > /proc/sys/fs/binfmt_misc/register
         COMMAND ${CMAKE_COMMAND} -E
           echo "binfmt_misc FEX-x86 installed"
         )
@@ -130,7 +119,7 @@ if (CMAKE_SYSTEM_PROCESSOR MATCHES "aarch64")
           echo "Attempting to install FEX-x86_64 misc now."
         COMMAND ${CMAKE_COMMAND} -E
           echo
-          ':FEX-x86_64:M:0:\\x7fELF\\x02\\x01\\x01\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x02\\x00\\x3e\\x00:\\xff\\xff\\xff\\xff\\xff\\xfe\\xfe\\x00\\x00\\x00\\x00\\xff\\xff\\xff\\xff\\xff\\xfe\\xff\\xff\\xff:${CMAKE_INSTALL_PREFIX}/bin/FEXInterpreter:${SUPPORTED_BINFMT_MISC_FLAGS}' > /proc/sys/fs/binfmt_misc/register
+          ':FEX-x86_64:M:0:\\x7fELF\\x02\\x01\\x01\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x00\\x02\\x00\\x3e\\x00:\\xff\\xff\\xff\\xff\\xff\\xfe\\xfe\\x00\\x00\\x00\\x00\\xff\\xff\\xff\\xff\\xff\\xfe\\xff\\xff\\xff:${CMAKE_INSTALL_PREFIX}/bin/FEXInterpreter:POCF' > /proc/sys/fs/binfmt_misc/register
         COMMAND ${CMAKE_COMMAND} -E
           echo "binfmt_misc FEX-x86_64 installed"
         )

Source/Tools/LinuxEmulation/LinuxSyscalls/FileManagement.cpp

@@ -322,32 +322,6 @@ FileManager::FileManager(FEXCore::Context::Context* ctx)
     }
   }
 
-  // Check to see if this kernel exposes `/proc/self/interpreter`.
-  // In the case that it does then behaviour is different than without.
-  //
-  // When procfs/interpreter is supported (binfmt_misc flag enabled):
-  // - procfs/exe -> symlink to the correct executable just like when executing natively.
-  // - procfs/interpreter -> symlink to FEXInterpreter.
-  //
-  // FEX no longer needs to track accesses to procfs/exe which improves performance and also improves correctness.
-  //
-  // When procfs/interpreter is supported (binfmt_misc flag not enabled):
-  // When procfs/interpreter is NOT supported:
-  // - procfs/exe -> symlink to FEXInterpreter.
-  // - procfs/interpreter -> symlink doesn't exist.
-  //
-  // In either of these two cases, FEX still needs to track procfs/exe so we can't completely get away from it.
-  // This happens in a few edge cases
-  // - binfmt_misc not installed
-  // - binfmt_misc doesn't support enabling the new flag
-  // - executable called through FEXInterpreter directly
-  //   - Can happen because of directly executing the process through FEXIntepreter or through FEXBash.
-  char FilenameExe[PATH_MAX];
-  char FilenameInterpreter[PATH_MAX];
-  const auto ExeSymlinkPath = FHU::Symlinks::ResolveSymlink("/proc/self/exe", FilenameExe);
-  const auto InterpreterSymlinkPath = FHU::Symlinks::ResolveSymlink("/proc/self/interpreter", FilenameInterpreter);
-  SupportsProcFSInterpreter = !InterpreterSymlinkPath.empty() && ExeSymlinkPath != InterpreterSymlinkPath;
-
   UpdatePID(::getpid());
 }
 
@@ -498,11 +472,6 @@ bool FileManager::IsSelfNoFollow(const char* Pathname, int flags) const {
 }
 
 std::optional<std::string_view> FileManager::GetSelf(const char* Pathname) {
-  if (SupportsProcFSInterpreter) {
-    // FEX doesn't need to track procfs/exe if this is supported.
-    return Pathname;
-  }
-
   if (!Pathname) {
     return std::nullopt;
   }
@@ -666,17 +635,15 @@ uint64_t FileManager::FAccessat2(int dirfd, const char* pathname, int mode, int
 }
 
 uint64_t FileManager::Readlink(const char* pathname, char* buf, size_t bufsiz) {
-  if (!SupportsProcFSInterpreter) {
-    // calculate the non-self link to exe
-    // Some executables do getpid, stat("/proc/$pid/exe")
-    char PidSelfPath[50];
-    snprintf(PidSelfPath, 50, "/proc/%i/exe", CurrentPID);
+  // calculate the non-self link to exe
+  // Some executables do getpid, stat("/proc/$pid/exe")
+  char PidSelfPath[50];
+  snprintf(PidSelfPath, 50, "/proc/%i/exe", CurrentPID);
 
-    if (strcmp(pathname, "/proc/self/exe") == 0 || strcmp(pathname, "/proc/thread-self/exe") == 0 || strcmp(pathname, PidSelfPath) == 0) {
-      const auto& App = Filename();
-      strncpy(buf, App.c_str(), bufsiz);
-      return std::min(bufsiz, App.size());
-    }
+  if (strcmp(pathname, "/proc/self/exe") == 0 || strcmp(pathname, "/proc/thread-self/exe") == 0 || strcmp(pathname, PidSelfPath) == 0) {
+    const auto& App = Filename();
+    strncpy(buf, App.c_str(), bufsiz);
+    return std::min(bufsiz, App.size());
   }
 
   FDPathTmpData TmpFilename;
@@ -745,15 +712,13 @@ uint64_t FileManager::Readlinkat(int dirfd, const char* pathname, char* buf, siz
     }
   }
 
-  if (!SupportsProcFSInterpreter) {
-    char PidSelfPath[50];
-    snprintf(PidSelfPath, 50, "/proc/%i/exe", CurrentPID);
+  char PidSelfPath[50];
+  snprintf(PidSelfPath, 50, "/proc/%i/exe", CurrentPID);
 
-    if (Path == "/proc/self/exe" || Path == "/proc/thread-self/exe" || Path == PidSelfPath) {
-      const auto& App = Filename();
-      strncpy(buf, App.c_str(), bufsiz);
-      return std::min(bufsiz, App.size());
-    }
+  if (Path == "/proc/self/exe" || Path == "/proc/thread-self/exe" || Path == PidSelfPath) {
+    const auto& App = Filename();
+    strncpy(buf, App.c_str(), bufsiz);
+    return std::min(bufsiz, App.size());
   }
 
   FDPathTmpData TmpFilename;

Source/Tools/LinuxEmulation/LinuxSyscalls/FileManagement.h

@@ -89,10 +89,6 @@ public:
   using FDPathTmpData = std::array<char[PATH_MAX], 2>;
   std::pair<int, const char*> GetEmulatedFDPath(int dirfd, const char* pathname, bool FollowSymlink, FDPathTmpData& TmpFilename);
 
-  bool SupportsProcFSInterpreterPath() const {
-    return SupportsProcFSInterpreter;
-  }
-
 #if defined(ASSERTIONS_ENABLED) && ASSERTIONS_ENABLED
   void TrackFEXFD(int FD) noexcept {
     std::lock_guard lk(FEXTrackingFDMutex);
@@ -166,6 +162,5 @@ private:
   FEX_CONFIG_OPT(Is64BitMode, IS64BIT_MODE);
   uint32_t CurrentPID {};
   int RootFSFD {AT_FDCWD};
-  bool SupportsProcFSInterpreter {};
 };
 } // namespace FEX::HLE

Source/Tools/LinuxEmulation/LinuxSyscalls/Syscalls.cpp

@@ -205,7 +205,6 @@ uint64_t ExecveHandler(FEXCore::Core::CpuStateFrame* Frame, const char* pathname
 
   // AT_EMPTY_PATH is only used if the pathname is empty.
   const bool IsFDExec = (Args.flags & AT_EMPTY_PATH) && strlen(pathname) == 0;
-  const bool SupportsProcFSInterpreter = SyscallHandler->FM.SupportsProcFSInterpreterPath();
   fextl::string FDExecEnv;
   fextl::string FDSeccompEnv;
 
@@ -233,19 +232,17 @@ uint64_t ExecveHandler(FEXCore::Core::CpuStateFrame* Frame, const char* pathname
       return -ENOENT;
     }
 
-    if (!SupportsProcFSInterpreter) {
-      int pid = getpid();
+    int pid = getpid();
 
-      char PidSelfPath[50];
-      snprintf(PidSelfPath, 50, "/proc/%i/exe", pid);
+    char PidSelfPath[50];
+    snprintf(PidSelfPath, 50, "/proc/%i/exe", pid);
 
-      if (strcmp(pathname, "/proc/self/exe") == 0 || strcmp(pathname, "/proc/thread-self/exe") == 0 || strcmp(pathname, PidSelfPath) == 0) {
-        // If the application is trying to execve `/proc/self/exe` or its variants,
-        // then we need to redirect this path to the true application path.
-        // This is because this path is a symlink to the executing application, which is always `FEXInterpreter` or `FEXLoader`.
-        // ex: JRE and shapez.io do this self-execution.
-        Filename = SyscallHandler->Filename();
-      }
+    if (strcmp(pathname, "/proc/self/exe") == 0 || strcmp(pathname, "/proc/thread-self/exe") == 0 || strcmp(pathname, PidSelfPath) == 0) {
+      // If the application is trying to execve `/proc/self/exe` or its variants,
+      // then we need to redirect this path to the true application path.
+      // This is because this path is a symlink to the executing application, which is always `FEXInterpreter` or `FEXLoader`.
+      // ex: JRE and shapez.io do this self-execution.
+      Filename = SyscallHandler->Filename();
     }
 
     Type = ELFLoader::ELFContainer::GetELFType(Filename);
@@ -392,8 +389,7 @@ uint64_t ExecveHandler(FEXCore::Core::CpuStateFrame* Frame, const char* pathname
     ExecveArgs.emplace_back(nullptr);
   }
 
-  const char* InterpreterPath = SupportsProcFSInterpreter ? "/proc/self/interpreter" : "/proc/self/exe";
-  Result = ::syscall(SYS_execveat, Args.dirfd, InterpreterPath, const_cast<char* const*>(ExecveArgs.data()), EnvpPtr, Args.flags);
+  Result = ::syscall(SYS_execveat, Args.dirfd, "/proc/self/exe", const_cast<char* const*>(ExecveArgs.data()), EnvpPtr, Args.flags);
   CloseSeccompFD();
   CloseFDExecFD();