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

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

[Philippe Mathieu-Daudé]

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.

 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);
         if (fd < 0) {
+            fprintf(stderr, "Error opening %s: %s\n", filename, gerr->message);
             return fd;
         }
         unlink(filename);
-- 
2.21.1

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

[Marc-André Lureau]

Hi

On Thu, Feb 27, 2020 at 11:06 AM Philippe Mathieu-Daudé
<philmd@redhat.com> 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.

G_DEFINE_AUTOPTR_CLEANUP_FUNC(GError, g_error_free)
it will call g_error_free() at the end of the scope if the variable is != NULL.

>
>  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);
>          if (fd < 0) {
> +            fprintf(stderr, "Error opening %s: %s\n", filename, gerr->message);

I am not sure if it's a good idea to printf() here, this may be
confused with the program output being run.

However, having a good errno value is probably necessary. And here,
glib doesn't guarantee that for this function, since it relies on
GFileError. So you would need something like g_file_error_to_errno()
which doesn't exist...



>              return fd;
>          }
>          unlink(filename);
> --
> 2.21.1
>

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

[Daniel P. Berrangé]

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

>              return fd;
>          }
>          unlink(filename);

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

[Ján Tomko]

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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
>-- 
>|: https://berrange.com      -o-    https://www.flickr.com/photos/dberrange :|
>|: https://libvirt.org         -o-            https://fstop138.berrange.com :|
>|: https://entangle-photo.org    -o-    https://www.instagram.com/dberrange :|
>
>

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