Le 21/06/2021 à 04:02, Takashi Yamamoto a écrit :
> On Sun, Jun 20, 2021 at 11:14 PM Laurent Vivier <laur...@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 <yamam...@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?
It's related to the text below.
The binary can be executable ('x') but not readable ('r'), so QEMU cannot load
it.
It's the purpose of the 'O' flag: kernel opens the file and pass the FD to QEMU
to execute it.
Thanks,
Laurent
>
>>
>> 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
