Patchwork qemu_timedate_diff() shouldn't modify its argument.

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Submitter Gleb Natapov
Date Nov. 6, 2011, 4 p.m.
Message ID <20111106160022.GF3225@redhat.com>
Download mbox | patch
Permalink /patch/123934/
State New
Headers show

Comments

Gleb Natapov - Nov. 6, 2011, 4 p.m.
The caller of qemu_timedate_diff() does not expect that tm it passes to
the function will be modified, but mktime() is destructive and modifies
its argument. Pass a copy of tm to it and set tm_isdst so that mktime()
will not rely on it since its value may be outdated.

Signed-off-by: Gleb Natapov <gleb@redhat.com>
--
			Gleb.
Rik van Riel - Nov. 7, 2011, 7:09 p.m.
On 11/06/2011 11:00 AM, Gleb Natapov wrote:
> The caller of qemu_timedate_diff() does not expect that tm it passes to
> the function will be modified, but mktime() is destructive and modifies
> its argument. Pass a copy of tm to it and set tm_isdst so that mktime()
> will not rely on it since its value may be outdated.

Ohhh, nice catch.

> Signed-off-by: Gleb Natapov<gleb@redhat.com>

Acked-by: Rik van Riel <riel@redhat.com>
Ronen Hod - Nov. 7, 2011, 8:16 p.m.
On 11/06/2011 06:00 PM, Gleb Natapov wrote:
> The caller of qemu_timedate_diff() does not expect that tm it passes to
> the function will be modified, but mktime() is destructive and modifies
> its argument. Pass a copy of tm to it and set tm_isdst so that mktime()
> will not rely on it since its value may be outdated.

I believe that the original issue was not related to outdated data at 
the moment of the daylight saving time transition.
using tmp.tm_isdst = -1 sounds good, but why use a copy of tm? The only 
significant field that will change in the tm is the tm_isdst itself that 
will be set to 0/1 (correctly).

Acked-by: Ronen Hod <rhod@redhat.com>

> Signed-off-by: Gleb Natapov<gleb@redhat.com>
> diff --git a/vl.c b/vl.c
> index 624da0f..641629b 100644
> --- a/vl.c
> +++ b/vl.c
> @@ -460,8 +460,11 @@ int qemu_timedate_diff(struct tm *tm)
>       if (rtc_date_offset == -1)
>           if (rtc_utc)
>               seconds = mktimegm(tm);
> -        else
> -            seconds = mktime(tm);
> +        else {
> +            struct tm tmp = *tm;
> +            tmp.tm_isdst = -1; /* use timezone to figure it out */
> +            seconds = mktime(&tmp);
> +	}
>       else
>           seconds = mktimegm(tm) + rtc_date_offset;
>
> --
> 			Gleb.
>
Gleb Natapov - Nov. 8, 2011, 7:36 a.m.
On Mon, Nov 07, 2011 at 10:16:29PM +0200, Ronen Hod wrote:
> On 11/06/2011 06:00 PM, Gleb Natapov wrote:
> >The caller of qemu_timedate_diff() does not expect that tm it passes to
> >the function will be modified, but mktime() is destructive and modifies
> >its argument. Pass a copy of tm to it and set tm_isdst so that mktime()
> >will not rely on it since its value may be outdated.
> 
> I believe that the original issue was not related to outdated data
> at the moment of the daylight saving time transition.
Don't know what do you mean here. The problem is definitely related to
incorrect data about daylight saving time.

> using tmp.tm_isdst = -1 sounds good, but why use a copy of tm? The
Hmm, what is not clear about "qemu_timedate_diff() shouldn't modify its
 argument"? :) 

> only significant field that will change in the tm is the tm_isdst
> itself that will be set to 0/1 (correctly).
This is incorrect (read man), but event if it would breed a new kind of
especially handsome ponies, if this is what caller wants it should call
mktime() by itself and not be a side effect of some random function
call.

> 
> Acked-by: Ronen Hod <rhod@redhat.com>
> 
> >Signed-off-by: Gleb Natapov<gleb@redhat.com>
> >diff --git a/vl.c b/vl.c
> >index 624da0f..641629b 100644
> >--- a/vl.c
> >+++ b/vl.c
> >@@ -460,8 +460,11 @@ int qemu_timedate_diff(struct tm *tm)
> >      if (rtc_date_offset == -1)
> >          if (rtc_utc)
> >              seconds = mktimegm(tm);
> >-        else
> >-            seconds = mktime(tm);
> >+        else {
> >+            struct tm tmp = *tm;
> >+            tmp.tm_isdst = -1; /* use timezone to figure it out */
> >+            seconds = mktime(&tmp);
> >+	}
> >      else
> >          seconds = mktimegm(tm) + rtc_date_offset;
> >
> >--
> >			Gleb.
> >

--
			Gleb.
Anthony Liguori - Nov. 8, 2011, 5:24 p.m.
On 11/06/2011 10:00 AM, Gleb Natapov wrote:
> The caller of qemu_timedate_diff() does not expect that tm it passes to
> the function will be modified, but mktime() is destructive and modifies
> its argument. Pass a copy of tm to it and set tm_isdst so that mktime()
> will not rely on it since its value may be outdated.
>
> Signed-off-by: Gleb Natapov<gleb@redhat.com>

Applied.  Thanks.

Regards,

Anthony Liguori

> diff --git a/vl.c b/vl.c
> index 624da0f..641629b 100644
> --- a/vl.c
> +++ b/vl.c
> @@ -460,8 +460,11 @@ int qemu_timedate_diff(struct tm *tm)
>       if (rtc_date_offset == -1)
>           if (rtc_utc)
>               seconds = mktimegm(tm);
> -        else
> -            seconds = mktime(tm);
> +        else {
> +            struct tm tmp = *tm;
> +            tmp.tm_isdst = -1; /* use timezone to figure it out */
> +            seconds = mktime(&tmp);
> +	}
>       else
>           seconds = mktimegm(tm) + rtc_date_offset;
>
> --
> 			Gleb.
>
>

Patch

diff --git a/vl.c b/vl.c
index 624da0f..641629b 100644
--- a/vl.c
+++ b/vl.c
@@ -460,8 +460,11 @@  int qemu_timedate_diff(struct tm *tm)
     if (rtc_date_offset == -1)
         if (rtc_utc)
             seconds = mktimegm(tm);
-        else
-            seconds = mktime(tm);
+        else {
+            struct tm tmp = *tm;
+            tmp.tm_isdst = -1; /* use timezone to figure it out */
+            seconds = mktime(&tmp);
+	}
     else
         seconds = mktimegm(tm) + rtc_date_offset;