Re: [Qemu-devel] [PATCH v2 1/4] linux-user: add option to intercept execve() syscalls

[Laurent Vivier]

Le 14/06/2016 à 21:26, Joel Holdsworth a écrit :
> From: Petros Angelatos <address@hidden>
> 
> In order for one to use QEMU user mode emulation under a chroot, it is
> required to use binfmt_misc. This can be avoided by QEMU never doing a
> raw execve() to the host system.
> 
> Introduce a new option, -execve, that uses the current QEMU interpreter
> to intercept execve().
> 
> qemu_execve() will prepend the interpreter path , similar to what
> binfmt_misc would do, and then pass the modified execve() to the host.
> 
> It is necessary to parse hashbang scripts in that function otherwise
> the kernel will try to run the interpreter of a script without QEMU and
> get an invalid exec format error.
> 
> Signed-off-by: Petros Angelatos <address@hidden>
> Tested-by: Laurent Vivier <address@hidden>
> Reviewed-by: Laurent Vivier <address@hidden>
> ---
>  linux-user/main.c    |  37 ++++++++++++++
>  linux-user/qemu.h    |   1 +
>  linux-user/syscall.c | 137 
> ++++++++++++++++++++++++++++++++++++++++++++++-----
>  3 files changed, 164 insertions(+), 11 deletions(-)
> 
> diff --git a/linux-user/main.c b/linux-user/main.c
> index f8a8764..429dc06 100644
> --- a/linux-user/main.c
> +++ b/linux-user/main.c
> @@ -18,6 +18,8 @@
>   */
>  #include "qemu/osdep.h"
>  #include "qemu-version.h"
> +
> +#include <sys/auxv.h>
>  #include <sys/mman.h>
>  #include <sys/syscall.h>
>  #include <sys/resource.h>
> @@ -79,6 +81,7 @@ static void usage(int exitcode);
>  
>  static const char *interp_prefix = CONFIG_QEMU_INTERP_PREFIX;
>  const char *qemu_uname_release;
> +const char *qemu_execve_path;
>  
>  /* XXX: on x86 MAP_GROWSDOWN only works if ESP <= address + 32, so
>     we allocate a bigger stack. Need a better solution, for example
> @@ -3947,6 +3950,38 @@ static void handle_arg_guest_base(const char *arg)
>      have_guest_base = 1;
>  }
>  
> +static void handle_arg_execve(const char *arg)
> +{
> +    const char *execfn;
> +    char buf[PATH_MAX];
> +    char *ret;
> +    int len;
> +
> +    /* try getauxval() */
> +    execfn = (const char *) getauxval(AT_EXECFN);
> +
> +    if (execfn != 0) {
> +        ret = realpath(execfn, buf);
> +
> +        if (ret != NULL) {
> +            qemu_execve_path = strdup(buf);
> +            return;
> +        }
> +    }
> +
> +    /* try /proc/self/exe */
> +    len = readlink("/proc/self/exe", buf, sizeof(buf) - 1);
> +
> +    if (len != -1) {
> +        buf[len] = '\0';
> +        qemu_execve_path = strdup(buf);
> +        return;
> +    }
> +
> +    fprintf(stderr, "qemu_execve: unable to determine intepreter's path\n");
> +    exit(EXIT_FAILURE);
> +}
> +
>  static void handle_arg_reserved_va(const char *arg)
>  {
>      char *p;
> @@ -4032,6 +4067,8 @@ static const struct qemu_argument arg_table[] = {
>       "uname",      "set qemu uname release string to 'uname'"},
>      {"B",          "QEMU_GUEST_BASE",  true,  handle_arg_guest_base,
>       "address",    "set guest_base address to 'address'"},
> +    {"execve",     "QEMU_EXECVE",      false, handle_arg_execve,
> +     "",           "use this interpreter when a process calls execve()"},
>      {"R",          "QEMU_RESERVED_VA", true,  handle_arg_reserved_va,
>       "size",       "reserve 'size' bytes for guest virtual address space"},
>      {"d",          "QEMU_LOG",         true,  handle_arg_log,
> diff --git a/linux-user/qemu.h b/linux-user/qemu.h
> index 56f29c3..567bcc1 100644
> --- a/linux-user/qemu.h
> +++ b/linux-user/qemu.h
> @@ -149,6 +149,7 @@ void init_task_state(TaskState *ts);
>  void task_settid(TaskState *);
>  void stop_all_tasks(void);
>  extern const char *qemu_uname_release;
> +extern const char *qemu_execve_path;
>  extern unsigned long mmap_min_addr;
>  
>  /* ??? See if we can avoid exposing so much of the loader internals.  */
> diff --git a/linux-user/syscall.c b/linux-user/syscall.c
> index 71ccbd9..a478f56 100644
> --- a/linux-user/syscall.c
> +++ b/linux-user/syscall.c
> @@ -106,6 +106,7 @@ int __clone2(int (*fn)(void *), void *child_stack_base,
>  #include <linux/rtnetlink.h>
>  #endif
>  #include <linux/audit.h>
> +#include <linux/binfmts.h>
>  #include "linux_loop.h"
>  #include "uname.h"
>  
> @@ -6659,6 +6660,128 @@ static target_timer_t get_timer_id(abi_long arg)
>      return timerid;
>  }
>  
> +/* qemu_execve() Must return target values and target errnos. */
> +static abi_long qemu_execve(char *filename, char *argv[],
> +                  char *envp[])
> +{
> +    char *i_arg = NULL, *i_name = NULL;
> +    char **new_argp;
> +    int argc, fd, ret, i, offset = 3;
> +    char *cp;
> +    char buf[BINPRM_BUF_SIZE];
> +
> +    /* normal execve case */
> +    if (qemu_execve_path == NULL || *qemu_execve_path == 0) {
> +        return get_errno(execve(filename, argv, envp));

You need a safe_execve() here, too.

Laurent