Merge pull request #2196 from Sonicadvance1/optimize_symlink_following

Linux: Improve performance of hot paths in path searching
FEX-Emu · Dec 5, 2022 · 66e0d46 · 66e0d46
2 parents 1dd5642 + 50cf74d
commit 66e0d46
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Showing 4 changed files with 28 additions and 12 deletions.
diff --git a/Source/Tests/ELFCodeLoader2.h b/Source/Tests/ELFCodeLoader2.h
@@ -180,19 +180,21 @@ class ELFCodeLoader2 final : public FEXCore::CodeLoader {
 
   static std::string ResolveRootfsFile(std::string const &File, std::string RootFS) {
     // If the path is relative then just run that
-    if (std::filesystem::path(File).is_relative()) {
+    if (File[0] != '/') {
       return File;
     }
 
     std::string RootFSLink = RootFS + File;
 
-    while (std::filesystem::is_symlink(RootFSLink)) {
+    char Filename[PATH_MAX];
+    while(FEX::HLE::IsSymlink(RootFSLink)) {
       // Do some special handling if the RootFS's linker is a symlink
       // Ubuntu's rootFS by default provides an absolute location symlink to the linker
       // Resolve this around back to the rootfs
-      auto SymlinkTarget = std::filesystem::read_symlink(RootFSLink);
-      if (SymlinkTarget.is_absolute()) {
-        RootFSLink = RootFS + SymlinkTarget.string();
+      auto SymlinkSize = FEX::HLE::GetSymlink(RootFSLink, Filename, PATH_MAX - 1);
+      if (SymlinkSize > 0 && Filename[0] == '/') {
+        RootFSLink = RootFS;
+        RootFSLink += std::string_view(Filename, SymlinkSize);
       }
       else {
         break;

diff --git a/Source/Tests/LinuxSyscalls/EmulatedFiles/EmulatedFiles.cpp b/Source/Tests/LinuxSyscalls/EmulatedFiles/EmulatedFiles.cpp
@@ -737,7 +737,7 @@ namespace FEX::EmulatedFile {
     }
 
     std::error_code ec;
-    bool exists = std::filesystem::exists(Path, ec);
+    bool exists = access(Path.c_str(), F_OK) == 0;
     if (ec) {
       return -1;
     }

diff --git a/Source/Tests/LinuxSyscalls/FileManagement.cpp b/Source/Tests/LinuxSyscalls/FileManagement.cpp
@@ -355,7 +355,6 @@ FileManager::~FileManager() {
 }
 
 std::string FileManager::GetEmulatedPath(const char *pathname, bool FollowSymlink) {
-  auto RootFSPath = LDPath();
   if (!pathname || // If no pathname
       pathname[0] != '/' || // If relative
       strcmp(pathname, "/") == 0) { // If we are getting root
@@ -367,17 +366,19 @@ std::string FileManager::GetEmulatedPath(const char *pathname, bool FollowSymlin
     return thunkOverlay->second;
   }
 
+  auto RootFSPath = LDPath();
   if (RootFSPath.empty()) { // If RootFS doesn't exist
     return {};
   }
 
   std::string Path = RootFSPath + pathname;
   if (FollowSymlink) {
-    std::error_code ec;
-    while(std::filesystem::is_symlink(Path, ec)) {
-      auto SymlinkTarget = std::filesystem::read_symlink(Path);
-      if (SymlinkTarget.is_absolute()) {
-        Path = RootFSPath + SymlinkTarget.string();
+    char Filename[PATH_MAX];
+    while(FEX::HLE::IsSymlink(Path)) {
+      auto SymlinkSize = FEX::HLE::GetSymlink(Path, Filename, PATH_MAX - 1);
+      if (SymlinkSize > 0 && Filename[0] == '/') {
+        Path = RootFSPath;
+        Path += std::string_view(Filename, SymlinkSize);
       }
       else {
         break;

diff --git a/Source/Tests/LinuxSyscalls/FileManagement.h b/Source/Tests/LinuxSyscalls/FileManagement.h
@@ -15,6 +15,7 @@ tags: LinuxSyscalls|common
 #include <stddef.h>
 #include <string>
 #include <sys/stat.h>
+#include <unistd.h>
 #include <vector>
 
 #include <unordered_map>
@@ -27,6 +28,18 @@ struct Context;
 }
 
 namespace FEX::HLE {
+[[maybe_unused]]
+static bool IsSymlink(const std::string &Filename) {
+  // Checks to see if a filepath is a symlink.
+  struct stat Buffer{};
+  int Result = lstat(Filename.c_str(), &Buffer);
+  return Result == 0 && S_ISLNK(Buffer.st_mode);
+}
+
+[[maybe_unused]]
+static ssize_t GetSymlink(const std::string &Filename, char *ResultBuffer, size_t ResultBufferSize) {
+  return readlink(Filename.c_str(), ResultBuffer, ResultBufferSize);
+}
 
 struct open_how;