From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:40670) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1WIvrG-0000D0-Ku for qemu-devel@nongnu.org; Thu, 27 Feb 2014 02:58:24 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1WIvrA-0006Ce-M8 for qemu-devel@nongnu.org; Thu, 27 Feb 2014 02:58:18 -0500 Received: from mx1.redhat.com ([209.132.183.28]:22941) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1WIvrA-0006CR-Bv for qemu-devel@nongnu.org; Thu, 27 Feb 2014 02:58:12 -0500 Received: from int-mx11.intmail.prod.int.phx2.redhat.com (int-mx11.intmail.prod.int.phx2.redhat.com [10.5.11.24]) by mx1.redhat.com (8.14.4/8.14.4) with ESMTP id s1R7wBuP016869 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=OK) for ; Thu, 27 Feb 2014 02:58:11 -0500 From: Markus Armbruster References: <1392163249-6983-1-git-send-email-quintela@redhat.com> Date: Thu, 27 Feb 2014 08:58:08 +0100 In-Reply-To: <1392163249-6983-1-git-send-email-quintela@redhat.com> (Juan Quintela's message of "Wed, 12 Feb 2014 01:00:49 +0100") Message-ID: <87a9dd0yy7.fsf@blackfin.pond.sub.org> MIME-Version: 1.0 Content-Type: text/plain Subject: Re: [Qemu-devel] [PATCH v2] qemu_file: use fwrite() correctly List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Juan Quintela Cc: qemu-devel@nongnu.org Juan Quintela writes: > fwrite() returns the number of items written. But when there is one > error, it can return a short write. > > In the particular bug that I was tracking, I did a migration to a > read-only filesystem. And it was able to finish the migration > correctly. fwrite() never returned a negative error code, the 1st > time it returns 0, after that it returns 4096. (migration writes > chunks of about 14000 bytes). And it was able to "complete" the > migration with success (yes, reading the file was a bit more > difficult). On the 1st fwrite() for the read-only filesystem, > it returns an errno of -EPIPE, that is exactly what has failed. > > To add insult to injury, if your amount of memory was big enough (12GB > on my case), it overwrote some important structure, and from them, > malloc failed. This check makes the problem go away. > > Signed-off-by: Juan Quintela > --- > > v2: a.k.a Paolo was right > > On the first call to fwrite() it returns 0, and errno is setup to > EPIPE, exactly what we wanted. > > Once here, improve the commit message. > > qemu-file.c | 9 ++++++++- > 1 file changed, 8 insertions(+), 1 deletion(-) > > diff --git a/qemu-file.c b/qemu-file.c > index 9473b67..e5ec798 100644 > --- a/qemu-file.c > +++ b/qemu-file.c > @@ -100,7 +100,14 @@ static int stdio_put_buffer(void *opaque, const uint8_t *buf, int64_t pos, > int size) > { > QEMUFileStdio *s = opaque; > - return fwrite(buf, 1, size, s->stdio_file); > + int res; > + > + res = fwrite(buf, 1, size, s->stdio_file); > + > + if (res != size) { > + return -errno; > + } > + return res; > } > > static int stdio_get_buffer(void *opaque, uint8_t *buf, int64_t pos, int size) This is what got an R-by from Eric and me. And this is what Peter pulled from Juan: diff --git a/qemu-file.c b/qemu-file.c index 9473b67..f074af1 100644 --- a/qemu-file.c +++ b/qemu-file.c @@ -100,7 +100,14 @@ static int stdio_put_buffer(void *opaque, const uint8_t *buf, int64_t pos, int size) { QEMUFileStdio *s = opaque; - return fwrite(buf, 1, size, s->stdio_file); + int res; + + res = fwrite(buf, 1, size, s->stdio_file); + + if (res != size) { + return -EIO; /* fake errno value */ + } + return res; } Follow-up fix sent.