Re: [PATCH v2] linux-user: Use memfd for open syscall emulation

[Rainer Müller]

On 29/07/2022 18.01, Richard Henderson wrote:
> On 7/29/22 08:49, Rainer Müller wrote:
>> +            /* create temporary file to map stat to */
>> +            tmpdir = getenv("TMPDIR");
>> +            if (!tmpdir)
>> +                tmpdir = "/tmp";
>> +            snprintf(filename, sizeof(filename),
>> "%s/qemu-open.XXXXXX", tmpdir);
>> +            fd = mkstemp(filename);
>> +            if (fd < 0) {
>> +                return fd;
>> +            }
> 
> We've been using g_file_open_tmp elsewhere; probably good to follow suit
> here.

That seemed reasonable at first, but with regards to error handling it
gets a bit complicated.

The suggested g_file_open_tmp() would leave us with a GError only, but
to return something meaningful to the caller we must set errno in this
context. As far as I can see, there is no way to convert back to an
errno from GError.

With g_file_open_tmp() we could always set the same generic errno, but
that would hide the real cause completely. I debugged this problem with
this message that was confusing, but at least it gave away a hint:
  cat: can't open '/proc/self/stat': Read-only file system

The other option would be to g_assert_true(fd >= 0) and kill the process
in case opening the temporary file failed. This also feels wrong, as the
caller could still recover from this state and continue.

Rainer