Fix clone (CLONE_VM) pid/tid reset (BZ#19957)

As discussed in libc-alpha [1] current clone with CLONE_VM (without
CLONE_THREAD set) will reset the pthread pid/tid fields to -1.  The
issue is since memory is shared between the parent and child it will
clobber parent's cached pid/tid leading to internal inconsistencies
if the value is not restored.

And even it is restored it may lead to racy conditions when between
set/restore a thread might invoke pthread function that validate the
pthread with INVALID_TD_P/INVALID_NOT_TERMINATED_TD_P and thus get
wrong results.

As stated in BZ19957, previously reports of this behaviour was close
with EWONTFIX due the fact usage of clone outside glibc is tricky
since glibc requires consistent internal pthread, while using clone
directly may not provide it. However since now posix_spawn uses
clone (CLONE_VM) to fixes various issues related to previous vfork
usage this issue requires fixing.

The vfork implementation also does something similar, but instead
it negates and restores only the *pid* field and functions that
might access its value know to handle such case (getpid, raise
and pthread ones that uses INVALID_TD_P/INVALID_NOT_TERMINATED_TD_P
macros that check only *tid* field).  Also vfork does not call
__clone directly, instead calling either __NR_vfork or __NR_clone
directly.

So this patch removes this clone behavior by avoiding setting
the pthread pid/tid field for CLONE_VM. There is no need to
check for CLONE_THREAD, since the minimum supported kernel in all
architecture implies that CLONE_VM must be used with CLONE_THREAD,
otherwise clone returns EINVAL.

Instead of current approach of:

   int clone(int (*fn)(void *), void *child_stack, int flags, ...)
      [...]
      if (flags & CLONE_THREAD)
        goto do_syscall;
      pid_t new_value;
      if (flags & CLONE_VM)
        new_value = -1;
      else
        new_value = getpid ();
      THREAD_SETMEM (THREAD_SELF, pid, new_value);
      THREAD_SETMEM (THREAD_SELF, tid, new_value);

    do_syscall:
      [...]

The new approach uses:

   int clone(int (*fn)(void *), void *child_stack, int flags, ...)
      [...]
      if (flags & CLONE_VM)
        goto do_syscall;
      pid_t new_value = getpid ();
      THREAD_SETMEM (THREAD_SELF, pid, new_value);
      THREAD_SETMEM (THREAD_SELF, tid, new_value);

    do_syscall:
      [...]

It also removes the linux tst-getpid2.c test which expects the previous
behavior and instead add another clone test.

Tested on x86_64, i686, x32, powerpc64le, aarch64, armhf, s390, and
s390x. I also did limited check on mips32 and sparc64 (using the new
added test).

I also got reviews from both m68k, hppa, and tile.  So I presume for
these architecture the patch works.

The fixes for alpha, microblaze, sh, ia64, and nio2 have not been
tested.

[1] https://sourceware.org/ml/libc-alpha/2016-04/msg00307.html

	* sysdeps/unix/sysv/linux/Makefile [$(subdir) == nptl] (test): Remove
	tst-getpid2.
	(test): Add tst-clone2.
	* sysdeps/unix/sysv/linux/tst-clone2.c: New file.
	* sysdeps/unix/sysv/linux/aarch64/clone.S (__clone): Do not change
	pid/tid fields for CLONE_VM.
	* sysdeps/unix/sysv/linux/arm/clone.S: Likewise.
	* sysdeps/unix/sysv/linux/i386/clone.S: Likewise.
	* sysdeps/unix/sysv/linux/mips/clone.S: Likewise.
	* sysdeps/unix/sysv/linux/powerpc/powerpc32/clone.S: Likewise.
	* sysdeps/unix/sysv/linux/powerpc/powerpc64/clone.S: Likewise.
	* sysdeps/unix/sysv/linux/s390/s390-32/clone.S: Likewise.
	* sysdeps/unix/sysv/linux/s390/s390-64/clone.S: Likewise.
	* sysdeps/unix/sysv/linux/sparc/sparc32/clone.S: Likewise.
	* sysdeps/unix/sysv/linux/sparc/sparc64/clone.S: Likewise.
	* sysdeps/unix/sysv/linux/x86_64/clone.S: Likewise.
	* sysdeps/unix/sysv/linux/tst-getpid2.c: Remove file.

21 files changed, 220 insertions, 141 deletions

diff --git a/sysdeps/unix/sysv/linux/Makefile b/sysdeps/unix/sysv/linux/Makefile
index 9999600..9b4e2e1 100644
--- a/sysdeps/unix/sysv/linux/Makefile
+++ b/sysdeps/unix/sysv/linux/Makefile
@@ -42,7 +42,7 @@ sysdep_headers += sys/mount.h sys/acct.h sys/sysctl.h \
 		  bits/socket_type.h bits/syscall.h bits/sysctl.h \
 		  bits/mman-linux.h
 
-tests += tst-clone tst-fanotify tst-personality
+tests += tst-clone tst-clone2 tst-fanotify tst-personality
 
 # Generate the list of SYS_* macros for the system calls (__NR_* macros).
 
@@ -194,7 +194,7 @@ CFLAGS-gai.c += -DNEED_NETLINK
 endif
 
 ifeq ($(subdir),nptl)
-tests += tst-setgetname tst-align-clone tst-getpid1 tst-getpid2 \
+tests += tst-setgetname tst-align-clone tst-getpid1 \
 	tst-thread-affinity-pthread tst-thread-affinity-pthread2 \
 	tst-thread-affinity-sched
 endif
diff --git a/sysdeps/unix/sysv/linux/aarch64/clone.S b/sysdeps/unix/sysv/linux/aarch64/clone.S
index 8f3ab40..76baa7a 100644
--- a/sysdeps/unix/sysv/linux/aarch64/clone.S
+++ b/sysdeps/unix/sysv/linux/aarch64/clone.S
@@ -72,17 +72,15 @@ thread_start:
 	cfi_undefined (x30)
 	mov	x29, 0
 
-	tbnz	x11, #CLONE_THREAD_BIT, 3f
-	mov	x0, #-1
-	tbnz	x11, #CLONE_VM_BIT, 2f
+	tbnz	x11, #CLONE_VM_BIT, 1f
+
 	mov	x8, #SYS_ify(getpid)
 	svc	0x0
-2:
 	mrs	x1, tpidr_el0
 	sub	x1, x1, #PTHREAD_SIZEOF
 	str	w0, [x1, #PTHREAD_PID_OFFSET]
 	str	w0, [x1, #PTHREAD_TID_OFFSET]
-3:
+1:
 
 	/* Pick the function arg and execute.  */
 	mov	x0, x12
diff --git a/sysdeps/unix/sysv/linux/alpha/clone.S b/sysdeps/unix/sysv/linux/alpha/clone.S
index 556aa63..6a3154f 100644
--- a/sysdeps/unix/sysv/linux/alpha/clone.S
+++ b/sysdeps/unix/sysv/linux/alpha/clone.S
@@ -24,7 +24,6 @@
 #include <bits/errno.h>
 
 #define CLONE_VM	0x00000100
-#define CLONE_THREAD	0x00010000
 
 /* int clone(int (*fn)(void *arg), void *child_stack, int flags,
 	     void *arg, pid_t *ptid, void *tls, pid_t *ctid);
@@ -94,7 +93,7 @@ thread_start:
 
 	/* Check and see if we need to reset the PID.  */
 	ldq	t0, 16(sp)
-	lda	t1, CLONE_THREAD
+	lda	t1, CLONE_VM
 	and	t0, t1, t2
 	beq	t2, 2f
 1:
@@ -123,11 +122,7 @@ thread_start:
 	.align	4
 2:
 	rduniq
-	lda	t1, CLONE_VM
 	mov	v0, s0
-	lda	v0, -1
-	and	t0, t1, t2
-	bne	t2, 3f
 	lda	v0, __NR_getxpid
 	callsys
 3:
diff --git a/sysdeps/unix/sysv/linux/arm/clone.S b/sysdeps/unix/sysv/linux/arm/clone.S
index c103123..7ff6818 100644
--- a/sysdeps/unix/sysv/linux/arm/clone.S
+++ b/sysdeps/unix/sysv/linux/arm/clone.S
@@ -70,19 +70,16 @@ PSEUDO_END (__clone)
 1:
 	.fnstart
 	.cantunwind
-	tst	ip, #CLONE_THREAD
-	bne	3f
+	tst	ip, #CLONE_VM
+	bne	2f
 	GET_TLS (lr)
 	mov	r1, r0
-	tst	ip, #CLONE_VM
 	ldr	r7, =SYS_ify(getpid)
-	ite	ne
-	movne	r0, #-1
-	swieq	0x0
+	swi	0x0
 	NEGOFF_ADJ_BASE (r1, TID_OFFSET)
 	str	r0, NEGOFF_OFF1 (r1, TID_OFFSET)
 	str	r0, NEGOFF_OFF2 (r1, PID_OFFSET, TID_OFFSET)
-3:
+2:
 	@ pick the function arg and call address off the stack and execute
 	ldr	r0, [sp, #4]
 	ldr 	ip, [sp], #8
diff --git a/sysdeps/unix/sysv/linux/hppa/clone.S b/sysdeps/unix/sysv/linux/hppa/clone.S
index 0a18137..3d037f1 100644
--- a/sysdeps/unix/sysv/linux/hppa/clone.S
+++ b/sysdeps/unix/sysv/linux/hppa/clone.S
@@ -133,19 +133,13 @@ ENTRY(__clone)
 
 .LthreadStart:
 # define CLONE_VM_BIT		23	/* 0x00000100  */
-# define CLONE_THREAD_BIT	15	/* 0x00010000  */
 	/* Load original clone flags.
-	   If CLONE_THREAD was passed, don't reset the PID/TID.
-	   If CLONE_VM was passed, we need to store -1 to PID/TID.
-	   If CLONE_VM and CLONE_THREAD were not set store the result
-	   of getpid to PID/TID.  */
+	   If CLONE_VM was passed, don't modify PID/TID.
+	   Otherwise store the result of getpid to PID/TID.  */
 	ldw	-56(%sp), %r26
-	bb,<,n	%r26, CLONE_THREAD_BIT, 1f
-	bb,<	%r26, CLONE_VM_BIT, 2f
-	ldi	-1, %ret0
+	bb,<,n	%r26, CLONE_VM_BIT, 1f
 	ble     0x100(%sr2, %r0)
 	ldi	__NR_getpid, %r20
-2:
 	mfctl	%cr27, %r26
 	stw	%ret0, PID_THREAD_OFFSET(%r26)
 	stw	%ret0, TID_THREAD_OFFSET(%r26)
diff --git a/sysdeps/unix/sysv/linux/i386/clone.S b/sysdeps/unix/sysv/linux/i386/clone.S
index ef447d1..25f2a9c 100644
--- a/sysdeps/unix/sysv/linux/i386/clone.S
+++ b/sysdeps/unix/sysv/linux/i386/clone.S
@@ -40,7 +40,6 @@
 #define SYS_clone 120
 
 #define CLONE_VM	0x00000100
-#define CLONE_THREAD	0x00010000
 
         .text
 ENTRY (__clone)
@@ -108,7 +107,7 @@ L(thread_start):
 	cfi_undefined (eip);
 	/* Note: %esi is zero.  */
 	movl	%esi,%ebp	/* terminate the stack frame */
-	testl	$CLONE_THREAD, %edi
+	testl	$CLONE_VM, %edi
 	je	L(newpid)
 L(haspid):
 	call	*%ebx
@@ -124,9 +123,6 @@ L(here):
 
 	.subsection 2
 L(newpid):
-	testl	$CLONE_VM, %edi
-	movl	$-1, %eax
-	jne	L(nomoregetpid)
 	movl	$SYS_ify(getpid), %eax
 	ENTER_KERNEL
 L(nomoregetpid):
diff --git a/sysdeps/unix/sysv/linux/ia64/clone2.S b/sysdeps/unix/sysv/linux/ia64/clone2.S
index fc046eb..b4cfdfc 100644
--- a/sysdeps/unix/sysv/linux/ia64/clone2.S
+++ b/sysdeps/unix/sysv/linux/ia64/clone2.S
@@ -67,12 +67,10 @@ ENTRY(__clone2)
 (CHILD)	mov loc0=gp
 (PARENT) ret
 	;;
-	tbit.nz p6,p0=in3,16	/* CLONE_THREAD */
-	tbit.z p7,p10=in3,8	/* CLONE_VM */
+	tbit.nz p6,p0=in3,8	/* CLONE_VM */
 (p6)	br.cond.dptk 1f
 	;;
 	mov r15=SYS_ify (getpid)
-(p10)	addl r8=-1,r0
 (p7)	break __BREAK_SYSCALL
 	;;
 	add r9=PID,r13
diff --git a/sysdeps/unix/sysv/linux/m68k/clone.S b/sysdeps/unix/sysv/linux/m68k/clone.S
index 33474cc..aec12cb 100644
--- a/sysdeps/unix/sysv/linux/m68k/clone.S
+++ b/sysdeps/unix/sysv/linux/m68k/clone.S
@@ -25,7 +25,6 @@
 #include <tls.h>
 
 #define CLONE_VM      0x00000100
-#define CLONE_THREAD  0x00010000
 
 /* int clone(int (*fn)(void *arg), void *child_stack, int flags, void *arg,
 	     void *parent_tidptr, void *tls, void *child_tidptr) */
@@ -100,16 +99,10 @@ thread_start:
 	cfi_undefined (pc)	/* Mark end of stack */
 	subl	%fp, %fp	/* terminate the stack frame */
 	/* Check and see if we need to reset the PID.  */
-	movel	%d1, %a1
-	andl	#CLONE_THREAD, %d1
-	jne	donepid
-	movel	%a1, %d1
-	movel	#-1, %d0
 	andl	#CLONE_VM, %d1
-	jne	gotpid
+	jne	donepid
 	movel	#SYS_ify (getpid), %d0
 	trap	#0
-gotpid:
 	movel	%a0, -(%sp)
 	movel	%d0, -(%sp)
 	bsrl	__m68k_read_tp@PLTPC
diff --git a/sysdeps/unix/sysv/linux/mips/clone.S b/sysdeps/unix/sysv/linux/mips/clone.S
index feb8250..39634c5 100644
--- a/sysdeps/unix/sysv/linux/mips/clone.S
+++ b/sysdeps/unix/sysv/linux/mips/clone.S
@@ -131,9 +131,8 @@ L(thread_start):
 	/* The stackframe has been created on entry of clone().  */
 
 	/* Check and see if we need to reset the PID.  */
-	LONG_L		a0,(PTRSIZE*2)(sp)
-	and		a1,a0,CLONE_THREAD
-	beqz		a1,L(restore_pid)
+	and	a1,a0,CLONE_VM
+	beqz	a1,L(restore_pid)
 L(donepid):
 
 	/* Restore the arg for user's function.  */
@@ -153,12 +152,8 @@ L(donepid):
 #endif
 
 L(restore_pid):
-	and		a1,a0,CLONE_VM
-	li		v0,-1
-	bnez		a1,L(gotpid)
 	li		v0,__NR_getpid
 	syscall
-L(gotpid):
 	READ_THREAD_POINTER(v1)
 	INT_S		v0,PID_OFFSET(v1)
 	INT_S		v0,TID_OFFSET(v1)
diff --git a/sysdeps/unix/sysv/linux/nios2/clone.S b/sysdeps/unix/sysv/linux/nios2/clone.S
index e4d3970..30b6e4a 100644
--- a/sysdeps/unix/sysv/linux/nios2/clone.S
+++ b/sysdeps/unix/sysv/linux/nios2/clone.S
@@ -26,7 +26,6 @@
 #include <tcb-offsets.h>
 
 #define CLONE_VM      0x00000100
-#define CLONE_THREAD  0x00010000
 
 /* int clone(int (*fn)(void *arg), void *child_stack, int flags, void *arg,
 	     void *parent_tidptr, void *tls, void *child_tidptr) */
@@ -71,13 +70,9 @@ thread_start:
 
 	/* We expect the argument registers to be preserved across system
 	   calls and across task cloning, so flags should be in r4 here.  */
-	andhi	r2, r4, %hi(CLONE_THREAD)
+	andi	r2, r4, CLONE_VM
 	bne	r2, zero, 2f
-	andi	r3, r4, CLONE_VM
-	movi	r2, -1
-	bne	r3, zero, 3f
         DO_CALL (getpid, 0)
-3:
 	stw	r2, PID_OFFSET(r23)
 	stw	r2, TID_OFFSET(r23)
 2:
diff --git a/sysdeps/unix/sysv/linux/powerpc/powerpc32/clone.S b/sysdeps/unix/sysv/linux/powerpc/powerpc32/clone.S
index eb973db..9d496f0 100644
--- a/sysdeps/unix/sysv/linux/powerpc/powerpc32/clone.S
+++ b/sysdeps/unix/sysv/linux/powerpc/powerpc32/clone.S
@@ -76,13 +76,11 @@ ENTRY (__clone)
 	crandc	cr1*4+eq,cr1*4+eq,cr0*4+so
 	bne-	cr1,L(parent)		/* The '-' is to minimise the race.  */
 
-	andis.	r0,r28,CLONE_THREAD>>16
-	bne+	r0,L(oldpid)
-	andi.	r0,r28,CLONE_VM
-	li	r3,-1
-	bne-	r0,L(nomoregetpid)
+	/* If CLONE_VM is set do not update the pid/tid field.  */
+	andi.	r0,r29,CLONE_VM
+	bne+	cr0,L(oldpid)
+
 	DO_CALL(SYS_ify(getpid))
-L(nomoregetpid):
 	stw	r3,TID(r2)
 	stw	r3,PID(r2)
 L(oldpid):
diff --git a/sysdeps/unix/sysv/linux/powerpc/powerpc64/clone.S b/sysdeps/unix/sysv/linux/powerpc/powerpc64/clone.S
index 959fdb7..7c59b9b 100644
--- a/sysdeps/unix/sysv/linux/powerpc/powerpc64/clone.S
+++ b/sysdeps/unix/sysv/linux/powerpc/powerpc64/clone.S
@@ -78,13 +78,11 @@ ENTRY (__clone)
 	crandc	cr1*4+eq,cr1*4+eq,cr0*4+so
 	bne-	cr1,L(parent)		/* The '-' is to minimise the race.  */
 
-	andis.	r0,r29,CLONE_THREAD>>16
+	/* If CLONE_VM is set do not update the pid/tid field.  */
+	rldicl.	r0,r29,56,63		/* flags & CLONE_VM.  */
 	bne+	cr0,L(oldpid)
-	andi.	r0,r29,CLONE_VM
-	li	r3,-1
-	bne-	cr0,L(nomoregetpid)
+
 	DO_CALL(SYS_ify(getpid))
-L(nomoregetpid):
 	stw	r3,TID(r13)
 	stw	r3,PID(r13)
 L(oldpid):
diff --git a/sysdeps/unix/sysv/linux/s390/s390-32/clone.S b/sysdeps/unix/sysv/linux/s390/s390-32/clone.S
index f774e90..2f8fa0b 100644
--- a/sysdeps/unix/sysv/linux/s390/s390-32/clone.S
+++ b/sysdeps/unix/sysv/linux/s390/s390-32/clone.S
@@ -54,13 +54,10 @@ error:
 PSEUDO_END (__clone)
 
 thread_start:
-	tmh	%r3,1		/* CLONE_THREAD == 0x00010000 */
-	jne	1f
-	lhi	%r2,-1
 	tml	%r3,256		/* CLONE_VM == 0x00000100 */
-	jne	2f
+	jne	1f
 	svc	SYS_ify(getpid)
-2:	ear	%r3,%a0
+	ear	%r3,%a0
 	st	%r2,PID(%r3)
 	st	%r2,TID(%r3)
 1:
diff --git a/sysdeps/unix/sysv/linux/s390/s390-64/clone.S b/sysdeps/unix/sysv/linux/s390/s390-64/clone.S
index 928a881..fb81692 100644
--- a/sysdeps/unix/sysv/linux/s390/s390-64/clone.S
+++ b/sysdeps/unix/sysv/linux/s390/s390-64/clone.S
@@ -55,13 +55,10 @@ error:
 PSEUDO_END (__clone)
 
 thread_start:
-	tmh	%r3,1		/* CLONE_THREAD == 0x00010000 */
+	tmll	%r3,256		/* CLONE_VM == 0x00000100 */
 	jne	1f
-	lhi	%r2,-1
-	tml	%r3,256		/* CLONE_VM == 0x00000100 */
-	jne	2f
 	svc	SYS_ify(getpid)
-2:	ear	%r3,%a0
+	ear	%r3,%a0
 	sllg	%r3,%r3,32
 	ear	%r3,%a1
 	st	%r2,PID(%r3)
diff --git a/sysdeps/unix/sysv/linux/sh/clone.S b/sysdeps/unix/sysv/linux/sh/clone.S
index a73dd7d..4cd7df1 100644
--- a/sysdeps/unix/sysv/linux/sh/clone.S
+++ b/sysdeps/unix/sysv/linux/sh/clone.S
@@ -67,15 +67,11 @@ ENTRY(__clone)
 	/* terminate the stack frame */
 	mov	#0, r14
 	mov	r4, r0
-	shlr16	r0
-	tst	#1, r0			// CLONE_THREAD = (1 << 16)
+	shlr8	r0
+	tst	#1, r0			// CLONE_VM = (1 << 8)
 	bf/s	4f
 	 mov	r4, r0
 	/* new pid */
-	shlr8	r0
-	tst	#1, r0			// CLONE_VM = (1 << 8)
-	bf/s	3f
-	 mov	#-1, r0
 	mov	#+SYS_ify(getpid), r3
 	trapa	#0x15
 3:
diff --git a/sysdeps/unix/sysv/linux/sparc/sparc32/clone.S b/sysdeps/unix/sysv/linux/sparc/sparc32/clone.S
index 68f8202..d6c92f6 100644
--- a/sysdeps/unix/sysv/linux/sparc/sparc32/clone.S
+++ b/sysdeps/unix/sysv/linux/sparc/sparc32/clone.S
@@ -25,7 +25,6 @@
 #include <sysdep.h>
 
 #define CLONE_VM	0x00000100
-#define CLONE_THREAD	0x00010000
 
 /* int clone(int (*fn)(void *arg), void *child_stack, int flags, void *arg,
 	     pid_t *ptid, void *tls, pid_t *ctid); */
@@ -80,15 +79,10 @@ END(__clone)
 
 	.type	__thread_start,@function
 __thread_start:
-	sethi	%hi(CLONE_THREAD), %l0
-	andcc	%g4, %l0, %g0
+	andcc	%g4, CLONE_VM, %g0
 	bne	1f
-	 andcc	%g4, CLONE_VM, %g0
-	bne,a	2f
-	 mov	-1,%o0
 	set	__NR_getpid,%g1
 	ta	0x10
-2:
 	st	%o0,[%g7 + PID]
 	st	%o0,[%g7 + TID]
 1:
diff --git a/sysdeps/unix/sysv/linux/sparc/sparc64/clone.S b/sysdeps/unix/sysv/linux/sparc/sparc64/clone.S
index cecffa7..b0f6266 100644
--- a/sysdeps/unix/sysv/linux/sparc/sparc64/clone.S
+++ b/sysdeps/unix/sysv/linux/sparc/sparc64/clone.S
@@ -25,7 +25,6 @@
 #include <sysdep.h>
 
 #define CLONE_VM	0x00000100
-#define CLONE_THREAD	0x00010000
 
 /* int clone(int (*fn)(void *arg), void *child_stack, int flags, void *arg,
 	     pid_t *ptid, void *tls, pid_t *ctid); */
@@ -77,15 +76,11 @@ END(__clone)
 
 	.type __thread_start,@function
 __thread_start:
-	sethi	%hi(CLONE_THREAD), %l0
-	andcc	%g4, %l0, %g0
+	andcc	%g4, CLONE_VM, %g0
 	bne,pt	%icc, 1f
-	 andcc	%g4, CLONE_VM, %g0
-	bne,a,pn %icc, 2f
-	 mov	-1,%o0
 	set	__NR_getpid,%g1
 	ta	0x6d
-2:	st	%o0,[%g7 + PID]
+	st	%o0,[%g7 + PID]
 	st	%o0,[%g7 + TID]
 1:
 	mov	%g0, %fp	/* terminate backtrace */
diff --git a/sysdeps/unix/sysv/linux/tile/clone.S b/sysdeps/unix/sysv/linux/tile/clone.S
index a300eb5..d1d3646 100644
--- a/sysdeps/unix/sysv/linux/tile/clone.S
+++ b/sysdeps/unix/sysv/linux/tile/clone.S
@@ -164,43 +164,14 @@ ENTRY (__clone)
 	cfi_def_cfa_offset (FRAME_SIZE)
 	cfi_undefined (lr)
 	/* Check and see if we need to reset the PID, which we do if
-	   CLONE_THREAD isn't set, i.e. we're not staying in the thread group.
-	   If CLONE_VM is set, we're doing some kind of thread-like clone,
-	   so we set the tid/pid to -1 to disable using the cached values
-	   in getpid().  Otherwise (if CLONE_VM isn't set), it's a
-	   fork-like clone, and we go ahead and write the cached values
+	   CLONE_VM isn't set, i.e. it's a fork-like clone with a new
+	   address space.  In that case we update the cached values
 	   from the true system pid (retrieved via __NR_getpid syscall).  */
-#ifdef __tilegx__
-	{
-	 moveli r0, hw1_last(CLONE_VM)
-	 moveli r1, hw1_last(CLONE_THREAD)
-	}
-	{
-	 shl16insli r0, r0, hw0(CLONE_VM)
-	 shl16insli r1, r1, hw0(CLONE_THREAD)
-	}
-#else
-	{
-	 moveli r0, lo16(CLONE_VM)
-	 moveli r1, lo16(CLONE_THREAD)
-	}
-	{
-	 auli r0, r0, ha16(CLONE_VM)
-	 auli r1, r1, ha16(CLONE_THREAD)
-	}
-#endif
-	{
-	 and r0, r30, r0
-	 and r1, r30, r1
-	}
-	BNEZ r1, .Lno_reset_pid   /* CLONE_THREAD is set */
-	{
-	 movei r0, -1
-	 BNEZ r0, .Lgotpid         /* CLONE_VM is set */
-	}
+	moveli r0, CLONE_VM
+	and r0, r30, r0
+	BNEZ r0, .Lno_reset_pid   /* CLONE_VM is set */
 	moveli TREG_SYSCALL_NR_NAME, __NR_getpid
 	swint1
-.Lgotpid:
 	ADDLI_PTR r2, tp, PID_OFFSET
 	{
 	 ST4 r2, r0
diff --git a/sysdeps/unix/sysv/linux/tst-clone2.c b/sysdeps/unix/sysv/linux/tst-clone2.c
new file mode 100644
index 0000000..68a7e6d
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/tst-clone2.c
@@ -0,0 +1,178 @@
+/* Test if CLONE_VM does not change pthread pid/tid field (BZ #19957)
+   Copyright (C) 2016 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#include <sched.h>
+#include <signal.h>
+#include <string.h>
+#include <stdio.h>
+#include <fcntl.h>
+#include <unistd.h>
+#include <stddef.h>
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdlib.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+
+#include <tls.h> /* for THREAD_* macros.  */
+
+static int sig;
+static int pipefd[2];
+
+static int
+f (void *a)
+{
+  close (pipefd[0]);
+
+  pid_t pid = THREAD_GETMEM (THREAD_SELF, pid);
+  pid_t tid = THREAD_GETMEM (THREAD_SELF, tid);
+
+  while (write (pipefd[1], &pid, sizeof pid) < 0)
+    continue;
+  while (write (pipefd[1], &tid, sizeof tid) < 0)
+    continue;
+
+  return 0;
+}
+
+
+static int
+clone_test (int clone_flags)
+{
+  sig = SIGRTMIN;
+  sigset_t ss;
+  sigemptyset (&ss);
+  sigaddset (&ss, sig);
+  if (sigprocmask (SIG_BLOCK, &ss, NULL) != 0)
+    {
+      printf ("sigprocmask failed: %m\n");
+      return 1;
+    }
+
+  if (pipe2 (pipefd, O_CLOEXEC))
+    {
+      printf ("sigprocmask failed: %m\n");
+      return 1;
+    }
+
+  pid_t ppid = getpid ();
+
+#ifdef __ia64__
+  extern int __clone2 (int (*__fn) (void *__arg), void *__child_stack_base,
+		       size_t __child_stack_size, int __flags,
+		       void *__arg, ...);
+  char st[256 * 1024] __attribute__ ((aligned));
+  pid_t p = __clone2 (f, st, sizeof (st), clone_flags, 0);
+#else
+  char st[128 * 1024] __attribute__ ((aligned));
+#if _STACK_GROWS_DOWN
+  pid_t p = clone (f, st + sizeof (st), clone_flags, 0);
+#elif _STACK_GROWS_UP
+  pid_t p = clone (f, st, clone_flags, 0);
+#else
+#error "Define either _STACK_GROWS_DOWN or _STACK_GROWS_UP"
+#endif
+#endif
+  close (pipefd[1]);
+
+  if (p == -1)
+    {
+      printf ("clone failed: %m\n");
+      return 1;
+    }
+
+  pid_t pid, tid;
+  if (read (pipefd[0], &pid, sizeof pid) != sizeof pid)
+    {
+      printf ("read pid failed: %m\n");
+      kill (p, SIGKILL);
+      return 1;
+    }
+  if (read (pipefd[0], &tid, sizeof tid) != sizeof tid)
+    {
+      printf ("read pid failed: %m\n");
+      kill (p, SIGKILL);
+      return 1;
+    }
+
+  close (pipefd[0]);
+
+  int ret = 0;
+
+  /* For CLONE_VM glibc clone implementation does not change the pthread
+     pid/tid field.  */
+  if ((clone_flags & CLONE_VM) == CLONE_VM)
+    {
+      if ((ppid != pid) || (ppid != tid))
+	{
+	  printf ("parent pid (%i) != received pid/tid (%i/%i)\n",
+		  (int)ppid, (int)pid, (int)tid);
+	  ret = 1;
+	}
+    }
+  /* For any other flag clone updates the new pthread pid and tid with
+     the clone return value.  */
+  else
+    {
+      if ((p != pid) || (p != tid))
+	{
+	  printf ("child pid (%i) != received pid/tid (%i/%i)\n",
+		  (int)p, (int)pid, (int)tid);
+	  ret = 1;
+	}
+    }
+
+  int e;
+  if (waitpid (p, &e, __WCLONE) != p)
+    {
+      puts ("waitpid failed");
+      kill (p, SIGKILL);
+      return 1;
+    }
+  if (!WIFEXITED (e))
+    {
+      if (WIFSIGNALED (e))
+	printf ("died from signal %s\n", strsignal (WTERMSIG (e)));
+      else
+	puts ("did not terminate correctly");
+      return 1;
+    }
+  if (WEXITSTATUS (e) != 0)
+    {
+      printf ("exit code %d\n", WEXITSTATUS (e));
+      return 1;
+    }
+
+  return ret;
+}
+
+int
+do_test (void)
+{
+  /* First, check that the clone implementation, without any flag, updates
+     the struct pthread to contain the new PID and TID.  */
+  int ret = clone_test (0);
+  /* Second, check that with CLONE_VM the struct pthread PID and TID fields
+     remain unmodified after the clone.  Any modifications would cause problem
+     for the parent as described in bug 19957.  */
+  ret += clone_test (CLONE_VM);
+  return ret;
+}
+
+#define TEST_FUNCTION do_test ()
+#include "../test-skeleton.c"
diff --git a/sysdeps/unix/sysv/linux/tst-getpid2.c b/sysdeps/unix/sysv/linux/tst-getpid2.c
deleted file mode 100644
index fc98cb6..0000000
--- a/sysdeps/unix/sysv/linux/tst-getpid2.c
+++ /dev/null
@@ -1,2 +0,0 @@
-#define TEST_CLONE_FLAGS CLONE_VM
-#include "tst-getpid1.c"
diff --git a/sysdeps/unix/sysv/linux/x86_64/clone.S b/sysdeps/unix/sysv/linux/x86_64/clone.S
index 1a50957..66f4b11 100644
--- a/sysdeps/unix/sysv/linux/x86_64/clone.S
+++ b/sysdeps/unix/sysv/linux/x86_64/clone.S
@@ -24,7 +24,6 @@
 #include <asm-syntax.h>
 
 #define CLONE_VM	0x00000100
-#define CLONE_THREAD	0x00010000
 
 /* The userland implementation is:
    int clone (int (*fn)(void *arg), void *child_stack, int flags, void *arg),
@@ -92,14 +91,11 @@ L(thread_start):
 	   the outermost frame obviously.  */
 	xorl	%ebp, %ebp
 
-	testq	$CLONE_THREAD, %rdi
+	andq	$CLONE_VM, %rdi
 	jne	1f
-	testq	$CLONE_VM, %rdi
-	movl	$-1, %eax
-	jne	2f
 	movl	$SYS_ify(getpid), %eax
 	syscall
-2:	movl	%eax, %fs:PID
+	movl	%eax, %fs:PID
 	movl	%eax, %fs:TID
 1: