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[PATCH 3.2 20/62] af_unix: Don't set err in unix_stream_read_generic unless there was an error

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3.2.79-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Rainer Weikusat <rweikusat@mobileactivedefense.com>

commit 1b92ee3d03af6643df395300ba7748f19ecdb0c5 upstream.

The present unix_stream_read_generic contains various code sequences of
the form

err = -EDISASTER;
if (<test>)
	goto out;

This has the unfortunate side effect of possibly causing the error code
to bleed through to the final

out:
	return copied ? : err;

and then to be wrongly returned if no data was copied because the caller
didn't supply a data buffer, as demonstrated by the program available at

http://pad.lv/1540731

Change it such that err is only set if an error condition was detected.

Fixes: 3822b5c2fc62 ("af_unix: Revert 'lock_interruptible' in stream receive code")
Reported-by: Joseph Salisbury <joseph.salisbury@canonical.com>
Signed-off-by: Rainer Weikusat <rweikusat@mobileactivedefense.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
 net/unix/af_unix.c | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -2059,13 +2059,15 @@ static int unix_stream_recvmsg(struct ki
 	int err = 0;
 	long timeo;
 
-	err = -EINVAL;
-	if (sk->sk_state != TCP_ESTABLISHED)
+	if (unlikely(sk->sk_state != TCP_ESTABLISHED)) {
+		err = -EINVAL;
 		goto out;
+	}
 
-	err = -EOPNOTSUPP;
-	if (flags&MSG_OOB)
+	if (unlikely(flags & MSG_OOB)) {
+		err = -EOPNOTSUPP;
 		goto out;
+	}
 
 	target = sock_rcvlowat(sk, flags&MSG_WAITALL, size);
 	timeo = sock_rcvtimeo(sk, noblock);
@@ -2107,9 +2109,11 @@ static int unix_stream_recvmsg(struct ki
 				goto unlock;
 
 			unix_state_unlock(sk);
-			err = -EAGAIN;
-			if (!timeo)
+			if (!timeo) {
+				err = -EAGAIN;
 				break;
+			}
+
 			mutex_unlock(&u->readlock);
 
 			timeo = unix_stream_data_wait(sk, timeo);

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[PATCH 3.2 00/62] 3.2.79-rc1 review Ben Hutchings <ben@decadent.org.uk> - 2016-03-29 22:20 +0200
  [PATCH 3.2 27/62] USB: option: add support for SIM7100E Ben Hutchings <ben@decadent.org.uk> - 2016-03-29 22:20 +0200
  [PATCH 3.2 13/62] xen/pciback: Check PF instead of VF for  PCI_COMMAND_MEMORY Ben Hutchings <ben@decadent.org.uk> - 2016-03-29 22:20 +0200
  [PATCH 3.2 12/62] libata: fix HDIO_GET_32BIT ioctl Ben Hutchings <ben@decadent.org.uk> - 2016-03-29 22:20 +0200
  [PATCH 3.2 52/62] ALSA: seq: oss: Don't drain at closing a client Ben Hutchings <ben@decadent.org.uk> - 2016-03-29 22:20 +0200
  [PATCH 3.2 41/62] ipr: Fix out-of-bounds null overwrite Ben Hutchings <ben@decadent.org.uk> - 2016-03-29 22:20 +0200
  [PATCH 3.2 20/62] af_unix: Don't set err in unix_stream_read_generic  unless there was an error Ben Hutchings <ben@decadent.org.uk> - 2016-03-29 22:20 +0200
  [PATCH 3.2 21/62] af_unix: Guard against other == sk in  unix_dgram_sendmsg Ben Hutchings <ben@decadent.org.uk> - 2016-03-29 22:20 +0200
  [PATCH 3.2 35/62] sunrpc/cache: fix off-by-one in qword_get() Ben Hutchings <ben@decadent.org.uk> - 2016-03-29 22:20 +0200
  [PATCH 3.2 42/62] ipr: Fix regression when loading firmware Ben Hutchings <ben@decadent.org.uk> - 2016-03-29 22:20 +0200
  [PATCH 3.2 15/62] xen/pcifront: Fix mysterious crashes when NUMA  locality information was extracted. Ben Hutchings <ben@decadent.org.uk> - 2016-03-29 22:20 +0200
  [PATCH 3.2 50/62] ASoC: wm8994: Fix enum ctl accesses in a wrong type Ben Hutchings <ben@decadent.org.uk> - 2016-03-29 22:20 +0200
  [PATCH 3.2 49/62] ASoC: wm8958: Fix enum ctl accesses in a wrong type Ben Hutchings <ben@decadent.org.uk> - 2016-03-29 22:20 +0200
  [PATCH 3.2 57/62] ubi: Fix out of bounds write in volume update code Ben Hutchings <ben@decadent.org.uk> - 2016-03-29 22:30 +0200
  [PATCH 3.2 03/62] iommu/vt-d: Fix 64-bit accesses to 32-bit  DMAR_GSTS_REG Ben Hutchings <ben@decadent.org.uk> - 2016-03-29 22:30 +0200
  [PATCH 3.2 45/62] ALSA: timer: Fix broken compat timer user  status ioctl Ben Hutchings <ben@decadent.org.uk> - 2016-03-29 22:30 +0200
  [PATCH 3.2 05/62] cfg80211/wext: fix message ordering Ben Hutchings <ben@decadent.org.uk> - 2016-03-29 22:30 +0200
  [PATCH 3.2 14/62] xen/pciback: Save the number of MSI-X entries  to be copied later. Ben Hutchings <ben@decadent.org.uk> - 2016-03-29 22:30 +0200
  [PATCH 3.2 18/62] tracing: Fix freak link error caused by branch  tracer Ben Hutchings <ben@decadent.org.uk> - 2016-03-29 22:30 +0200
  [PATCH 3.2 16/62] ALSA: seq: Fix leak of pool buffer at  concurrent writes Ben Hutchings <ben@decadent.org.uk> - 2016-03-29 22:30 +0200
  [PATCH 3.2 62/62] HID: usbhid: fix recursive deadlock Ben Hutchings <ben@decadent.org.uk> - 2016-03-29 22:30 +0200
  [PATCH 3.2 08/62] drm/i915: fix error path in intel_setup_gmbus() Ben Hutchings <ben@decadent.org.uk> - 2016-03-29 22:30 +0200
  [PATCH 3.2 19/62] ALSA: seq: Fix double port list deletion Ben Hutchings <ben@decadent.org.uk> - 2016-03-29 22:30 +0200
  [PATCH 3.2 58/62] Revert "drm/radeon: call hpd_irq_event on resume" Ben Hutchings <ben@decadent.org.uk> - 2016-03-29 22:30 +0200
  [PATCH 3.2 10/62] s390/dasd: prevent incorrect length error under  z/VM after PAV changes Ben Hutchings <ben@decadent.org.uk> - 2016-03-29 22:30 +0200
  [PATCH 3.2 07/62] nfs: fix nfs_size_to_loff_t Ben Hutchings <ben@decadent.org.uk> - 2016-03-29 22:30 +0200
  [PATCH 3.2 04/62] wext: fix message delay/ordering Ben Hutchings <ben@decadent.org.uk> - 2016-03-29 22:30 +0200
  [PATCH 3.2 06/62] mac80211: fix use of uninitialised values in RX  aggregation Ben Hutchings <ben@decadent.org.uk> - 2016-03-29 22:30 +0200
  [PATCH 3.2 46/62] ALSA: hdspm: Fix wrong boolean ctl value accesses Ben Hutchings <ben@decadent.org.uk> - 2016-03-29 22:30 +0200
  [PATCH 3.2 01/62] Revert "crypto: algif_skcipher - Do not  dereference ctx without socket lock" Ben Hutchings <ben@decadent.org.uk> - 2016-03-29 22:30 +0200
  [PATCH 3.2 17/62] tracepoints: Do not trace when cpu is offline Ben Hutchings <ben@decadent.org.uk> - 2016-03-29 22:30 +0200
  Re: [PATCH 3.2 00/62] 3.2.79-rc1 review Guenter Roeck <linux@roeck-us.net> - 2016-03-29 22:30 +0200
    Re: [PATCH 3.2 00/62] 3.2.79-rc1 review Ben Hutchings <ben@decadent.org.uk> - 2016-03-29 23:10 +0200
  [PATCH 3.2 02/62] crypto: {blk,giv}cipher: Set has_setkey Ben Hutchings <ben@decadent.org.uk> - 2016-03-29 22:30 +0200

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