From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:53390) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1er2Ae-0007Mj-C4 for qemu-devel@nongnu.org; Wed, 28 Feb 2018 08:53:26 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1er2Ad-0002ia-6p for qemu-devel@nongnu.org; Wed, 28 Feb 2018 08:53:24 -0500 Date: Wed, 28 Feb 2018 13:53:05 +0000 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Message-ID: <20180228135305.GM17774@redhat.com> Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= References: <20180228131315.30194-1-mreitz@redhat.com> <20180228131315.30194-2-mreitz@redhat.com> <20180228133434.GL17774@redhat.com> <1fc55643-7ca6-26a5-7e2e-7d6b09a43a13@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <1fc55643-7ca6-26a5-7e2e-7d6b09a43a13@redhat.com> Content-Transfer-Encoding: quoted-printable Subject: Re: [Qemu-devel] [PATCH 1/2] block/file-posix: Fix fully preallocated truncate List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Max Reitz Cc: qemu-block@nongnu.org, qemu-devel@nongnu.org, "eblake@redhat.com Kevin Wolf" , qemu-stable@nongnu.org On Wed, Feb 28, 2018 at 02:45:49PM +0100, Max Reitz wrote: > On 2018-02-28 14:34, Daniel P. Berrang=C3=A9 wrote: > > On Wed, Feb 28, 2018 at 02:13:14PM +0100, Max Reitz wrote: > >> Storing the lseek() result in an int results in it overflowing when = the > >> file is at least 2 GB big. Then, we have a 50 % chance of the resul= t > >> being "negative" and thus thinking an error occurred when actually > >> everything went just fine. > >> > >> So we should use the correct type for storing the result: off_t. > >> > >> Reported-by: Daniel P. Berrange > >> Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=3D1549231 > >> Cc: qemu-stable@nongnu.org > >> Signed-off-by: Max Reitz > >> --- > >> block/file-posix.c | 5 +++-- > >> 1 file changed, 3 insertions(+), 2 deletions(-) > >> > >> diff --git a/block/file-posix.c b/block/file-posix.c > >> index f1591c3849..90c25864a0 100644 > >> --- a/block/file-posix.c > >> +++ b/block/file-posix.c > >> @@ -1697,6 +1697,7 @@ static int raw_regular_truncate(int fd, int64_= t offset, PreallocMode prealloc, > >> case PREALLOC_MODE_FULL: > >> { > >> int64_t num =3D 0, left =3D offset - current_length; > >> + off_t seek_result; > >> =20 > >> /* > >> * Knowing the final size from the beginning could allow th= e file > >> @@ -1711,8 +1712,8 @@ static int raw_regular_truncate(int fd, int64_= t offset, PreallocMode prealloc, > >> =20 > >> buf =3D g_malloc0(65536); > >> =20 > >> - result =3D lseek(fd, current_length, SEEK_SET); > >> - if (result < 0) { > >> + seek_result =3D lseek(fd, current_length, SEEK_SET); > >> + if (seek_result < 0) { > >=20 > > off_t is an unsigned type, so this comparison to "< 0" is bogus - onl= y the > > exact value (off_t)-1 indicates an error. So this needs to be > >=20 > > if (seek_result =3D=3D (off_t)-1) { > > ... > > } >=20 > Hmmm... On my system, it appears to be a long int[1]. And > find_allocation() does an off_t < 0 comparison already. And > "man 0p sys_types.h" says "blkcnt_t and off_t shall be signed integer > types." Hmm, that's odd then - lseek man page explicitly said it must be cast, which suggested to me it could be unsigned: RETURN VALUE Upon successful completion, lseek() returns the resulting offset = loca=E2=80=90 tion as measured in bytes from the beginning of the file. On e= rror, the value (off_t) -1 is returned and errno is set to indicate= the error. CC'ing Eric for the "official" POSIX answer.... Regards, Daniel --=20 |: https://berrange.com -o- https://www.flickr.com/photos/dberran= ge :| |: https://libvirt.org -o- https://fstop138.berrange.c= om :| |: https://entangle-photo.org -o- https://www.instagram.com/dberran= ge :|