ProcFS: Reads past the end of a generated file should be zero-length

Kernel/FileSystem/ProcFS.cpp

@@ -1107,13 +1107,16 @@ ssize_t ProcFSInode::read_bytes(off_t offset, ssize_t count, u8* buffer, FileDes
     }
 
     auto& data = generated_data;
-    ssize_t nread = 0;
-    if (data.has_value()) {
-        nread = min(static_cast<off_t>(data.value().size() - offset), static_cast<off_t>(count));
-        memcpy(buffer, data.value().data() + offset, nread);
-        if (nread == 0 && description && description->generator_cache())
-            description->generator_cache().clear();
-    }
+    if (!data.has_value())
+        return 0;
+
+    if ((size_t)offset >= data.value().size())
+        return 0;
+
+    ssize_t nread = min(static_cast<off_t>(data.value().size() - offset), static_cast<off_t>(count));
+    memcpy(buffer, data.value().data() + offset, nread);
+    if (nread == 0 && description && description->generator_cache())
+        description->generator_cache().clear();
 
     return nread;
 }

Userland/test_io.cpp

@@ -140,6 +140,22 @@ void test_tmpfs_read_past_end()
     close(fd);
 }
 
+void test_procfs_read_past_end()
+{
+    int fd = open("/proc/uptime", O_RDONLY);
+    ASSERT(fd >= 0);
+
+    int rc = lseek(fd, 4096, SEEK_SET);
+    ASSERT(rc == 4096);
+
+    char buffer[16];
+    int nread = read(fd, buffer, sizeof(buffer));
+    if (nread != 0) {
+        fprintf(stderr, "Expected 0-length read past end of file in /proc\n");
+    }
+    close(fd);
+}
+
 int main(int, char**)
 {
     int rc;
@@ -160,6 +176,7 @@ int main(int, char**)
     test_ftruncate_negative();
     test_mmap_directory();
     test_tmpfs_read_past_end();
+    test_procfs_read_past_end();
 
     return 0;
 }