Implement write sealing

src/libraries/Common/src/Interop/Unix/System.Native/Interop.Fcntl.cs

@@ -22,6 +22,9 @@ internal static partial class Fcntl
             [LibraryImport(Libraries.SystemNative, EntryPoint = "SystemNative_FcntlSetFD", SetLastError = true)]
             internal static partial int SetFD(SafeHandle fd, int flags);
 
+            [LibraryImport(Libraries.SystemNative, EntryPoint = "SystemNative_FcntlSetSealWrite", SetLastError = true)]
+            internal static partial int SetSealWrite(SafeHandle fd);
+
             [LibraryImport(Libraries.SystemNative, EntryPoint = "SystemNative_FcntlGetFD", SetLastError = true)]
             internal static partial int GetFD(SafeHandle fd);
 

src/libraries/System.IO.MemoryMappedFiles/src/System/IO/MemoryMappedFiles/MemoryMappedFile.Unix.cs

@@ -233,7 +233,19 @@ private static SafeFileHandle CreateSharedBackingObject(Interop.Sys.MemoryMapped
 
             try
             {
-                if (!Interop.Sys.IsMemfdSupported)
+                if (Interop.Sys.IsMemfdSupported)
+                {
+                    // Add a writeseal for readonly case when eadonly protection requested
+                    if ((protections & Interop.Sys.MemoryMappedProtections.PROT_READ) != 0 &&
+                        (protections & Interop.Sys.MemoryMappedProtections.PROT_WRITE) == 0 &&
+                        // seal write failed
+                        Interop.Sys.Fcntl.SetSealWrite(fd) == -1)
+                    {
+                        fd.Dispose();
+                        throw Interop.GetExceptionForIoErrno(Interop.Sys.GetLastErrorInfo());
+                    }
+                }
+                else
                 {
                     // Unlink the shared memory object immediately so that it'll go away once all handles
                     // to it are closed (as with opened then unlinked files, it'll remain usable via
@@ -251,6 +263,7 @@ private static SafeFileHandle CreateSharedBackingObject(Interop.Sys.MemoryMapped
                 if (inheritability == HandleInheritability.Inheritable &&
                     Interop.Sys.Fcntl.SetFD(fd, 0) == -1)
                 {
+                    fd.Dispose();
                     throw Interop.GetExceptionForIoErrno(Interop.Sys.GetLastErrorInfo());
                 }
 

src/native/libs/System.Native/entrypoints.c

@@ -71,6 +71,7 @@ static const Entry s_sysNative[] =
     DllImportEntry(SystemNative_OpenDir)
     DllImportEntry(SystemNative_CloseDir)
     DllImportEntry(SystemNative_Pipe)
+    DllImportEntry(SystemNative_FcntlSetSealWrite)
     DllImportEntry(SystemNative_FcntlSetFD)
     DllImportEntry(SystemNative_FcntlGetFD)
     DllImportEntry(SystemNative_FcntlCanGetSetPipeSz)

src/native/libs/System.Native/pal_io.c

@@ -385,7 +385,7 @@ int32_t SystemNative_MemfdSupported(void)
 
     // Note that the name has no affect on file descriptor behavior. From linux manpage: 
     //   Names do not affect the behavior of the file descriptor, and as such multiple files can have the same name without any side effects.
-    int32_t fd = memfd_create("test", MFD_CLOEXEC);
+    int32_t fd = memfd_create("test", MFD_CLOEXEC | MFD_ALLOW_SEALING);
     if (fd < 0) return 0;
 
     close(fd);
@@ -405,7 +405,7 @@ intptr_t SystemNative_MemfdCreate(const char* name)
     assert(strlen(name) <= PATH_MAX);
 #endif
 
-    return memfd_create(name, MFD_CLOEXEC);
+    return memfd_create(name, MFD_CLOEXEC | MFD_ALLOW_SEALING);
 #else
     (void)name;
     errno = ENOTSUP;
@@ -665,6 +665,13 @@ int32_t SystemNative_Pipe(int32_t pipeFds[2], int32_t flags)
     return result;
 }
 
+int32_t SystemNative_FcntlSetSealWrite(intptr_t fd)
+{
+    int result;
+    while ((result = fcntl(ToFileDescriptor(fd), F_ADD_SEALS, F_SEAL_WRITE)) < 0 && errno == EINTR);
+    return result;
+}
+
 int32_t SystemNative_FcntlSetFD(intptr_t fd, int32_t flags)
 {
     int result;

src/native/libs/System.Native/pal_io.h

@@ -432,6 +432,13 @@ PALEXPORT int32_t SystemNative_Pipe(int32_t pipefd[2], // [out] pipefds[0] gets
 // for each command. This allows use to have strongly typed arguments and saves
 // complexity around converting command codes.
 
+/**
+ * Sets the write seal on memfd_create file descriptor.
+ *
+ * Returns 0 for success; -1 for failure. Sets errno for failure.
+ */
+PALEXPORT int32_t SystemNative_FcntlSetSealWrite(intptr_t fd);
+
 /**
  * Sets the O_CLOEXEC flag on a file descriptor.
  *