Re: [PATCH v1] linux-user: Add option to run `execve`d programs through QEMU

[Noah Goldstein <goldstein.w.n@gmail.com>]

On Tue, Oct 29, 2024 at 10:23 AM Alex Bennée <alex.bennee@linaro.org> wrote:
>
> Noah Goldstein <goldstein.w.n@gmail.com> writes:
>
> > The new option '-qemu-children' makes it so that on `execve` the child
> > process will be launch by the same `qemu` executable that is currently
> > running along with its current commandline arguments.
> >
> > The motivation for the change is to make it so that plugins running
> > through `qemu` can continue to run on children.  Why not just
> > `binfmt`?: Plugins can be desirable regardless of system/architecture
> > emulation, and can sometimes be useful for elf files that can run
> > natively. Enabling `binfmt` for all natively runnable elf files may
> > not be desirable.
>
> Broadly I don't see anything wrong with the patch. However my principle
> concern is how test it. Currently nothing in check-tcg ensures this
> mechanism works and stresses the argument copying.

Fair enough. I will add some tests.
>
> >
> > Signed-off-by: Noah Goldstein <goldstein.w.n@gmail.com>
> > ---
> >  linux-user/main.c           | 22 ++++++++++++++++++++++
> >  linux-user/syscall.c        | 20 +++++++++++++++-----
> >  linux-user/user-internals.h |  4 ++++
> >  3 files changed, 41 insertions(+), 5 deletions(-)
> >
> > diff --git a/linux-user/main.c b/linux-user/main.c
> > index 8143a0d4b0..dfb303a1f2 100644
> > --- a/linux-user/main.c
> > +++ b/linux-user/main.c
> > @@ -81,6 +81,10 @@ unsigned long mmap_min_addr;
> >  uintptr_t guest_base;
> >  bool have_guest_base;
> >
> > +bool qemu_dup_for_children;
> > +int qemu_argc;
> > +char ** qemu_argv;
> > +
> >  /*
> >   * Used to implement backwards-compatibility for the `-strace`, and
> >   * QEMU_STRACE options. Without this, the QEMU_LOG can be overwritten by
> > @@ -451,6 +455,11 @@ static void handle_arg_jitdump(const char *arg)
> >      perf_enable_jitdump();
> >  }
> >
> > +static void handle_arg_qemu_children(const char *arg)
> > +{
> > +    qemu_dup_for_children = true;
> > +}
> > +
> >  static QemuPluginList plugins = QTAILQ_HEAD_INITIALIZER(plugins);
> >
> >  #ifdef CONFIG_PLUGIN
> > @@ -526,6 +535,10 @@ static const struct qemu_argument arg_table[] = {
> >       "",           "Generate a /tmp/perf-${pid}.map file for perf"},
> >      {"jitdump",    "QEMU_JITDUMP",     false, handle_arg_jitdump,
> >       "",           "Generate a jit-${pid}.dump file for perf"},
> > +    {"qemu-children",
> > +                   "QEMU_CHILDREN",    false, handle_arg_qemu_children,
> > +     "",           "Run child processes (created with execve) with qemu "
> > +                   "(as instantiated for the parent)"},
> >      {NULL, NULL, false, NULL, NULL, NULL}
> >  };
> >
> > @@ -729,6 +742,15 @@ int main(int argc, char **argv, char **envp)
> >
> >      optind = parse_args(argc, argv);
> >
> > +    if (qemu_dup_for_children) {
> > +        int i;
> > +        qemu_argc = optind;
> > +        qemu_argv = g_new0(char *, qemu_argc);
> > +        for (i = 0; i < optind; ++i) {
> > +            qemu_argv[i] = strdup(argv[i]);
> > +        }
> > +    }
> > +
> >      qemu_set_log_filename_flags(last_log_filename,
> >                                  last_log_mask | (enable_strace * LOG_STRACE),
> >                                  &error_fatal);
> > diff --git a/linux-user/syscall.c b/linux-user/syscall.c
> > index 9d5415674d..732ef89054 100644
> > --- a/linux-user/syscall.c
> > +++ b/linux-user/syscall.c
> > @@ -8459,13 +8459,14 @@ static int do_execv(CPUArchState *cpu_env, int dirfd,
> >                      abi_long pathname, abi_long guest_argp,
> >                      abi_long guest_envp, int flags, bool is_execveat)
> >  {
> > -    int ret;
> > +    int ret, argp_offset;
> >      char **argp, **envp;
> >      int argc, envc;
> >      abi_ulong gp;
> >      abi_ulong addr;
> >      char **q;
> >      void *p;
> > +    bool through_qemu = !is_execveat && qemu_dup_for_children;
> >
> >      argc = 0;
> >
> > @@ -8489,10 +8490,11 @@ static int do_execv(CPUArchState *cpu_env, int dirfd,
> >          envc++;
> >      }
> >
> > -    argp = g_new0(char *, argc + 1);
> > +    argp_offset = through_qemu ? qemu_argc : 0;
> > +    argp = g_new0(char *, argc + argp_offset + 1);
> >      envp = g_new0(char *, envc + 1);
> >
> > -    for (gp = guest_argp, q = argp; gp; gp += sizeof(abi_ulong), q++) {
> > +    for (gp = guest_argp, q = argp + argp_offset; gp; gp += sizeof(abi_ulong), q++) {
> >          if (get_user_ual(addr, gp)) {
> >              goto execve_efault;
> >          }
> > @@ -8537,9 +8539,17 @@ static int do_execv(CPUArchState *cpu_env, int dirfd,
> >      }
> >
> >      const char *exe = p;
> > -    if (is_proc_myself(p, "exe")) {
> > +    if (through_qemu) {
> > +        int i;
> > +        for (i = 0; i < argp_offset; ++i) {
> > +            argp[i] = qemu_argv[i];
> > +        }
> > +        exe = qemu_argv[0];
> > +    }
> > +    else if (is_proc_myself(p, "exe")) {
> >          exe = exec_path;
> >      }
> > +
> >      ret = is_execveat
> >          ? safe_execveat(dirfd, exe, argp, envp, flags)
> >          : safe_execve(exe, argp, envp);
> > @@ -8553,7 +8563,7 @@ execve_efault:
> >      ret = -TARGET_EFAULT;
> >
> >  execve_end:
> > -    for (gp = guest_argp, q = argp; *q; gp += sizeof(abi_ulong), q++) {
> > +    for (gp = guest_argp, q = argp + argp_offset; *q; gp += sizeof(abi_ulong), q++) {
> >          if (get_user_ual(addr, gp) || !addr) {
> >              break;
> >          }
> > diff --git a/linux-user/user-internals.h b/linux-user/user-internals.h
> > index 5c7f173ceb..0719e65ff4 100644
> > --- a/linux-user/user-internals.h
> > +++ b/linux-user/user-internals.h
> > @@ -30,6 +30,10 @@ void stop_all_tasks(void);
> >  extern const char *qemu_uname_release;
> >  extern unsigned long mmap_min_addr;
> >
> > +extern bool qemu_dup_for_children;
> > +extern int qemu_argc;
> > +extern char ** qemu_argv;
> > +
> >  typedef struct IOCTLEntry IOCTLEntry;
> >
> >  typedef abi_long do_ioctl_fn(const IOCTLEntry *ie, uint8_t *buf_temp,
>
> --
> Alex Bennée
> Virtualisation Tech Lead @ Linaro