Re: [RFC PATCH] linux-user/syscall: Use g_file_open_tmp()

["Ján Tomko" <jtomko@redhat.com>]

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On a Thursday in 2020, Daniel P. Berrangé wrote:
>On Thu, Feb 27, 2020 at 11:06:21AM +0100, Philippe Mathieu-Daudé wrote:
>> Use GLib g_file_open_tmp() instead of getenv + snprintf + mkstemp.
>>
>> Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
>> ---
>> RFC because I'm not sure g_autoptr(GError) works this way.
>
>It does work. Any struct that's defined in GLib has support for
>g_autoptr(). If you aren't suyre though, just check for a
>G_DEFINE_AUTOPTR_CLEANUP_FUNC() macro usage that refers to the
>struct in question
>
>$ grep -r 'G_DEFINE_AUTOPTR_CLEANUP_FUNC(GError' /usr/include/glib-2.0
>/usr/include/glib-2.0/glib/glib-autocleanups.h:G_DEFINE_AUTOPTR_CLEANUP_FUNC(GError, g_error_free)
>
>>  linux-user/syscall.c | 11 ++++-------
>>  1 file changed, 4 insertions(+), 7 deletions(-)
>>
>> diff --git a/linux-user/syscall.c b/linux-user/syscall.c
>> index 8d27d10807..0e44969e16 100644
>> --- a/linux-user/syscall.c
>> +++ b/linux-user/syscall.c
>> @@ -7282,17 +7282,14 @@ static int do_openat(void *cpu_env, int dirfd, const char *pathname, int flags,
>>      }
>>
>>      if (fake_open->filename) {
>> -        const char *tmpdir;
>> -        char filename[PATH_MAX];
>> +        g_autoptr(GError) gerr = NULL;
>> +        g_autofree gchar *filename = NULL;
>>          int fd, r;
>>
>>          /* 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);
>> +        fd = g_file_open_tmp("qemu-open.XXXXXX", &filename, &gerr);
>
>g_file_open_tmp, calls g_get_tmp_name, which calls
>g_get_tmp_dir, which defaults to $TMPDIR, falling back
>to /tmp. So we're using the same dir as before.
>
>>          if (fd < 0) {
>> +            fprintf(stderr, "Error opening %s: %s\n", filename, gerr->message);
>
>This is wrong - the returned "filename" is only valid when
>g_file_open_tmp succeeds. So the use of "filename" here
>is likely a NULL. Given that the only place you use "filename"
>is in the error path, and that's not valid, we can simply
>eliminate it entirely, and pass NULL into g_file_open_tmp
>

'filename' is used right below to unlink it.
But it can be dropped from the error message here since it's included in the
error reported by GLib.

Jano

>>              return fd;
>>          }
>>          unlink(filename);
>
>Regards,
>Daniel
>-- 
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>

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