linux-user: add support of binfmt_misc 'O' flag

The binfmt_misc module can calculate the credentials and security
token according to the binary instead of to the interpreter if the
'C' flag is enabled.

To be able to execute non-readable binaries, this flag implies 'O'
flag. When 'O' flag is enabled, bintfmt_misc opens the file for
reading and pass the file descriptor to the interpreter.

References:
linux/Documentation/binfmt_misc.txt          ['O' and 'C' description]
linux/fs/binfmt_misc.c linux/fs/binfmt_elf.c [ AT_EXECFD usage ]

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>

linux-user/linuxload.c

@@ -131,7 +131,7 @@ abi_ulong loader_build_argptr(int envc, int argc, abi_ulong sp,
     return sp;
 }
 
-int loader_exec(const char * filename, char ** argv, char ** envp,
+int loader_exec(int fdexec, const char *filename, char **argv, char **envp,
              struct target_pt_regs * regs, struct image_info *infop,
              struct linux_binprm *bprm)
 {
@@ -140,11 +140,7 @@ int loader_exec(const char * filename, char ** argv, char ** envp,
 
     bprm->p = TARGET_PAGE_SIZE*MAX_ARG_PAGES-sizeof(unsigned int);
     memset(bprm->page, 0, sizeof(bprm->page));
-    retval = open(filename, O_RDONLY);
+    bprm->fd = fdexec;
-    if (retval < 0) {
-        return -errno;
-    }
-    bprm->fd = retval;
     bprm->filename = (char *)filename;
     bprm->argc = count(argv);
     bprm->argv = argv;

linux-user/main.c

@@ -3618,6 +3618,26 @@ static int parse_args(int argc, char **argv)
     return optind;
 }
 
+static int get_execfd(char **envp)
+{
+    typedef struct {
+        long a_type;
+        long a_val;
+    } auxv_t;
+    auxv_t *auxv;
+
+    while (*envp++ != NULL) {
+        ;
+    }
+
+    for (auxv = (auxv_t *)envp; auxv->a_type != AT_NULL; auxv++) {
+        if (auxv->a_type == AT_EXECFD) {
+            return auxv->a_val;
+        }
+    }
+    return -1;
+}
+
 int main(int argc, char **argv, char **envp)
 {
     struct target_pt_regs regs1, *regs = &regs1;
@@ -3632,6 +3652,7 @@ int main(int argc, char **argv, char **envp)
     int target_argc;
     int i;
     int ret;
+    int execfd;
 
     module_call_init(MODULE_INIT_QOM);
 
@@ -3809,7 +3830,16 @@ int main(int argc, char **argv, char **envp)
     env->opaque = ts;
     task_settid(ts);
 
-    ret = loader_exec(filename, target_argv, target_environ, regs,
+    execfd = get_execfd(envp);
+    if (execfd < 0) {
+        execfd = open(filename, O_RDONLY);
+    }
+    if (execfd < 0) {
+        printf("Error while loading %s: %s\n", filename, strerror(-execfd));
+        _exit(1);
+    }
+
+    ret = loader_exec(execfd, filename, target_argv, target_environ, regs,
         info, &bprm);
     if (ret != 0) {
         printf("Error while loading %s: %s\n", filename, strerror(-ret));

linux-user/qemu.h

@@ -174,7 +174,7 @@ struct linux_binprm {
 void do_init_thread(struct target_pt_regs *regs, struct image_info *infop);
 abi_ulong loader_build_argptr(int envc, int argc, abi_ulong sp,
                               abi_ulong stringp, int push_ptr);
-int loader_exec(const char * filename, char ** argv, char ** envp,
+int loader_exec(int fdexec, const char *filename, char **argv, char **envp,
              struct target_pt_regs * regs, struct image_info *infop,
              struct linux_binprm *);