Re: [PATCH v2 01/11] linux-user: handle /proc/self/exe for execve

[Takashi Yamamoto]

On Sun, Jun 20, 2021 at 11:14 PM Laurent Vivier <laurent@vivier.eu> wrote:
>
> Le 31/05/2021 à 07:50, YAMAMOTO Takashi a écrit :
> > It seems somehow common to execve /proc/self/exe in docker
> > or golang community these days.
> > At least, moby "reexec" and runc "libcontainer" do that.
> >
> > Signed-off-by: YAMAMOTO Takashi <yamamoto@midokura.com>
> > ---
> >  linux-user/syscall.c | 7 ++++++-
> >  1 file changed, 6 insertions(+), 1 deletion(-)
> >
> > diff --git a/linux-user/syscall.c b/linux-user/syscall.c
> > index c9f812091c..a2b03ecb8b 100644
> > --- a/linux-user/syscall.c
> > +++ b/linux-user/syscall.c
> > @@ -8470,6 +8470,7 @@ static abi_long do_syscall1(void *cpu_env, int num, 
> > abi_long arg1,
> >  #endif
> >      case TARGET_NR_execve:
> >          {
> > +            const char *path;
> >              char **argp, **envp;
> >              int argc, envc;
> >              abi_ulong gp;
> > @@ -8537,7 +8538,11 @@ static abi_long do_syscall1(void *cpu_env, int num, 
> > abi_long arg1,
> >               * before the execve completes and makes it the other
> >               * program's problem.
> >               */
> > -            ret = get_errno(safe_execve(p, argp, envp));
> > +            path = p;
> > +            if (is_proc_myself(path, "exe")) {
> > +                path = exec_path;
> > +            }
> > +            ret = get_errno(safe_execve(path, argp, envp));
> >              unlock_user(p, arg1, 0);
> >
> >              goto execve_end;
> >
>
> The problem here is QEMU can fail to execute the file directly.

i don't understand this sentence. can you explain a bit?

>
> The binary can be launched with binfmt_misc and the 'O' flag:
>
>      ``O`` - open-binary
>             Legacy behavior of binfmt_misc is to pass the full path
>             of the binary to the interpreter as an argument. When this flag is
>             included, binfmt_misc will open the file for reading and pass its
>             descriptor as an argument, instead of the full path, thus allowing
>             the interpreter to execute non-readable binaries. This feature
>             should be used with care - the interpreter has to be trusted not 
> to
>             emit the contents of the non-readable binary.
>
> You should use do_openat() (that resolves the /proc/self/exe path) and 
> fexecve().

i thought there was an issue with the approach. but i don't remember
what it was.
maybe i will retry it.

>
> Thanks,
> Laurent