kernel: Support setuid binaries

You still have to run "chmod 4755 /bin/su" as root inside Luna for now, as this is not done by the install scripts.

apps/su.cpp

@@ -1,11 +1,12 @@
 #include <os/ArgumentParser.h>
+#include <pwd.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <unistd.h>
 
 int main(int argc, char** argv)
 {
-    StringView id;
+    StringView name;
 
     if (geteuid() != 0)
     {
@@ -14,19 +15,24 @@ int main(int argc, char** argv)
     }
 
     os::ArgumentParser parser;
-    parser.add_positional_argument(id, "id"_sv, true);
+    parser.add_positional_argument(name, "name"_sv, true);
     parser.parse(argc, argv);
 
-    int uid = atoi(id.chars());
-
-    if (uid == 0)
+    struct passwd* entry = getpwnam(name.chars());
+    if (!entry)
     {
-        fprintf(stderr, "Already root!\n");
+        fprintf(stderr, "su: user %s not found!\n", name.chars());
         return 1;
     }
 
-    setgid(uid);
-    setuid(uid);
-
-    execl("/bin/sh", "sh", NULL);
+    if (getuid() != geteuid())
+    {
+        fprintf(stderr, "FIXME: you have to enter %s's password first!\n", name.chars());
+        return 1;
+    }
+
+    setgid(entry->pw_gid);
+    setuid(entry->pw_uid);
+
+    execl(entry->pw_shell, entry->pw_shell, NULL);
 }

initrd/etc/passwd

@@ -0,0 +1,2 @@
+root:toor:0:0:Administrator:/:/bin/sh
+selene:moon:1000:1000:User:/home/selene:/bin/sh

kernel/src/fs/VFS.cpp

@@ -86,4 +86,14 @@ namespace VFS
 
         return inode->mode() & S_IROTH;
     }
+
+    bool is_setuid(SharedPtr<Inode> inode)
+    {
+        return inode->mode() & S_ISUID;
+    }
+
+    bool is_setgid(SharedPtr<Inode> inode)
+    {
+        return inode->mode() & S_ISGID;
+    }
 }

kernel/src/fs/VFS.h

@@ -178,6 +178,8 @@ namespace VFS
     bool can_execute(SharedPtr<Inode> inode, Credentials auth);
     bool can_read(SharedPtr<Inode> inode, Credentials auth);
     bool can_write(SharedPtr<Inode> inode, Credentials auth);
+    bool is_setuid(SharedPtr<Inode> inode);
+    bool is_setgid(SharedPtr<Inode> inode);
 
     Inode& root_inode();
 }

kernel/src/sys/exec.cpp

@@ -76,6 +76,9 @@ Result<u64> sys_execve(Registers* regs, SyscallArgs args)
 
     MMU::delete_userspace_page_directory(current->directory);
 
+    if (VFS::is_setuid(inode)) current->auth.euid = current->auth.suid = inode->uid();
+    if (VFS::is_setgid(inode)) current->auth.egid = current->auth.sgid = inode->gid();
+
     current->name = path.chars();
 
     image->apply(current);

libc/CMakeLists.txt

@@ -16,6 +16,7 @@ set(SOURCES
     src/dirent.cpp
     src/setjmp.cpp
     src/env.cpp
+    src/pwd.cpp
     src/sys/stat.cpp
     src/sys/mman.cpp
     src/sys/wait.cpp

libc/include/pwd.h

@@ -0,0 +1,43 @@
+/* pwd.h: Password file parsing. */
+
+#ifndef _PWD_H
+#define _PWD_H
+
+#include <sys/types.h>
+
+struct passwd
+{
+    char* pw_name;
+    char* pw_passwd;
+    uid_t pw_uid;
+    gid_t pw_gid;
+    char* pw_gecos;
+    char* pw_dir;
+    char* pw_shell;
+};
+
+#ifdef __cplusplus
+extern "C"
+{
+#endif
+
+    /* Read the next entry from the password file. */
+    struct passwd* getpwent(void);
+
+    /* Find the entry with a matching username in the password file. */
+    struct passwd* getpwnam(const char* name);
+
+    /* Find the entry with a matching user ID in the password file. */
+    struct passwd* getpwuid(uid_t uid);
+
+    /* Rewind the password file. */
+    void setpwent(void);
+
+    /* End password file parsing. */
+    void endpwent(void);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif

libc/src/pwd.cpp

@@ -0,0 +1,109 @@
+#include <pwd.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+static struct passwd pwd;
+static FILE* f { nullptr };
+static char buf[4096];
+
+extern "C"
+{
+    struct passwd* getpwent()
+    {
+        if (!f)
+        {
+            f = fopen("/etc/passwd", "r");
+            if (!f) return nullptr;
+        }
+
+        while (true)
+        {
+            char* rc = fgets(buf, sizeof(buf), f);
+            if (!rc) return nullptr;
+
+            char* name = strtok(rc, ":\n");
+            if (!name) continue;
+
+            char* passwd = strtok(nullptr, ":\n");
+            if (!passwd) continue;
+
+            char* uid = strtok(nullptr, ":\n");
+            if (!uid) continue;
+
+            char* gid = strtok(nullptr, ":\n");
+            if (!gid) continue;
+
+            char* gecos = strtok(nullptr, ":\n");
+            if (!gecos) continue;
+
+            char* dir = strtok(nullptr, ":\n");
+            if (!dir) continue;
+
+            char* shell = strtok(nullptr, ":\n");
+            if (!shell) continue;
+
+            pwd.pw_name = name;
+            pwd.pw_passwd = passwd;
+            pwd.pw_uid = (uid_t)strtoul(uid, NULL, 10);
+            pwd.pw_gid = (gid_t)strtoul(gid, NULL, 10);
+            pwd.pw_gecos = gecos;
+            pwd.pw_dir = dir;
+            pwd.pw_shell = shell;
+
+            return &pwd;
+        }
+    }
+
+    struct passwd* getpwnam(const char* name)
+    {
+        setpwent();
+
+        struct passwd* entry;
+
+        while ((entry = getpwent()))
+        {
+            if (!strcmp(entry->pw_name, name))
+            {
+                endpwent();
+                return entry;
+            }
+        }
+
+        endpwent();
+        return entry;
+    }
+
+    struct passwd* getpwuid(uid_t uid)
+    {
+        setpwent();
+
+        struct passwd* entry;
+
+        while ((entry = getpwent()))
+        {
+            if (entry->pw_uid == uid)
+            {
+                endpwent();
+                return entry;
+            }
+        }
+
+        endpwent();
+        return entry;
+    }
+
+    void setpwent()
+    {
+        if (f) rewind(f);
+    }
+
+    void endpwent()
+    {
+        if (f)
+        {
+            fclose(f);
+            f = nullptr;
+        }
+    }
+}