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

[Laurent Vivier]

Le 22/01/2016 11:01, Petros Angelatos a écrit :
>>> diff --git a/linux-user/main.c b/linux-user/main.c
>>> index ee12035..5951279 100644
>>> --- a/linux-user/main.c
>>> +++ b/linux-user/main.c
>>> @@ -79,6 +79,7 @@ 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
>>> @@ -3828,6 +3829,11 @@ static void handle_arg_guest_base(const char *arg)
>>>      have_guest_base = 1;
>>>  }
>>>
>>> +static void handle_arg_execve(const char *arg)
>>> +{
>>> +    qemu_execve_path = strdup(arg);
>>
>> I think you can use the parameter just as an on/off switch and
>> realpath(argv[0]) as qemu_execve_path.
>>
>> I don't see any reason to use other binary than the one in use.
> 
> This was my initial approach too, but argv[0] can be just the filename
> like "qemu-arm-static". And while I could add extra logic to look this
> up in the PATH, someone could run it from a completely different
> location. Then I looked for a way to get the path of the current
> executable but every platform has its own way of doing that and I
> didn't want to add all these cases.
> 
> https://stackoverflow.com/questions/1023306/finding-current-executables-path-without-proc-self-exe

linux-user works only on linux.
qemu uses glib-2.0, so you can use g_find_program_in_path().

>>> diff --git a/linux-user/syscall.c b/linux-user/syscall.c
>>> index 0cbace4..d0b5442 100644
>>> --- a/linux-user/syscall.c
>>> +++ b/linux-user/syscall.c
>>> @@ -5854,6 +5854,109 @@ static target_timer_t get_timer_id(abi_long arg)
>>>      return timerid;
>>>  }
>>>
>>> +#define BINPRM_BUF_SIZE 128
>>
>> This is defined in <linux/binfmts.h>
> 
> Got it, I'll add this header and remove the definition.
> 
>>
>>> +/* 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];
>>> +
>>> +    for (argc = 0; argv[argc] != NULL; argc++) {
>>> +        /* nothing */ ;
>>> +    }
>>> +
>>> +    fd = open(filename, O_RDONLY);
>>> +    if (fd == -1) {
>>> +        return -ENOENT;
>>
>> return -errno; ?
> 
> Will fix in v2
> 
>>> +    ret = read(fd, buf, BINPRM_BUF_SIZE);
>>> +    if (ret == -1) {
>>> +        close(fd);
>>> +        return -ENOENT;
>>
>> return -errno; ?
> 
> Will fix in v2
> 
>>> +    }
>>> +
>>> +    close(fd);
>>> +
>>> +    /* adapted from the kernel
>>> +     * 
>>> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/fs/binfmt_script.c
>>> +     */
>>> +    if ((buf[0] == '#') && (buf[1] == '!')) {
>>
>> what happens if read() < 2 ?
> 
> Hm, the easy option is for qemu_execve to return ENOEXEC or EIO.
> Otherwise I could retry the read N times? I'm not sure how to handle
> this if we don't want to return an error.

if we have less than 2 bytes, we can guess it is not executable...

>>> +    /* Copy the original arguments with offset */
>>> +    for (i = 0; i < argc; i++) {
>>> +        new_argp[i + offset] = argv[i];
>>> +    }
>>> +
>>> +    new_argp[0] = strdup(qemu_execve_path);
>>> +    new_argp[1] = strdup("-0");
>>> +    new_argp[offset] = filename;
>>> +    new_argp[argc + offset] = NULL;
>>> +
>>> +    if (i_name) {
>>> +        new_argp[2] = i_name;
>>> +        new_argp[3] = i_name;
>>> +
>>> +        if (i_arg) {
>>> +            new_argp[4] = i_arg;
>>> +        }
>>> +    } else {
>>> +        new_argp[2] = argv[0];
>>> +    }
>>> +
>>> +    return get_errno(execve(qemu_execve_path, new_argp, envp));
>>
>> duplicate get_errno() with the caller.
> 
> I'll add the logic proposed bellow in this function and remove the
> duplicate get_errno() from the caller.
> 
>>>  /* do_syscall() should always have a single exit point at the end so
>>>     that actions, such as logging of syscall results, can be performed.
>>>     All errnos that do_syscall() returns must be -TARGET_<errcode>. */
>>> @@ -6113,7 +6216,13 @@ abi_long do_syscall(void *cpu_env, int num, abi_long 
>>> arg1,
>>>
>>>              if (!(p = lock_user_string(arg1)))
>>>                  goto execve_efault;
>>> -            ret = get_errno(execve(p, argp, envp));
>>> +
>>> +            if (qemu_execve_path && *qemu_execve_path) {
>>> +                ret = get_errno(qemu_execve(p, argp, envp));
>>> +            } else {
>>> +                ret = get_errno(execve(p, argp, envp));
>>> +            }
>>> +
>>
>> what do you think of:
>>
>>     ret = qemu_execve(p, argp, envp);
>>
>> and in qemu_execve():
>>
>>     if (qemu_execve_path == NULL || *qemu_execve_path == 0) {
>>         return get_errno(execve(p, argp, envp));
>>     }
>>
>> so all the logic is in the function.
> 
> Sounds good, I'll include this in v2
> 
> Since I'm new to this style of contribution I have a couple of

http://wiki.qemu.org/Contribute/SubmitAPatch

> questions. Is it ok that I deleted part of the patch for my reply to
> code review, or should I have replied inline without deleting

Generally, it's better to not delete parts. So, someone tacking the mail
thread at a moment can read the whole history in the last mail.

> anything? Should I send v2 after we resolve all the issues here or
> should I send a v2 with proposed fixes but lacking the ones pending
> replies?

Do as you want, but more you send versions, more you (can) have reviews.

BTW, I'm not the linux-user maintainer (Riku is), so I can just review
and comment, no more.

> 
> Thanks,
> Petros

Laurent