[PATCH 0/5] x86: uaccess CMPXCHG + KVM bug fixes

[PATCH 0/5] x86: uaccess CMPXCHG + KVM bug fixes

[Sean Christopherson]

Add uaccess macros for doing CMPXCHG on userspace addresses and use the
macros to fix KVM bugs by replacing flawed code that maps memory into the
kernel address space without proper mmu_notifier protection (or with
broken pfn calculations in one case).

Add yet another Kconfig for guarding asm_volatile_goto() to workaround a
clang-13 bug.  I've verified the test passes on gcc versions of arm64,
PPC, RISC-V, and s390x that also pass the CC_HAS_ASM_GOTO_OUTPUT test.

Patches 1-4 are tagged for stable@ as patches 3 and 4 (mostly 3) need a
backportable fix, and doing CMPXCHG on the userspace address is the
simplest fix from a KVM perspective.

Peter Zijlstra (1):
  x86/uaccess: Implement macros for CMPXCHG on user addresses

Sean Christopherson (4):
  Kconfig: Add option for asm goto w/ tied outputs to workaround
    clang-13 bug
  KVM: x86: Use __try_cmpxchg_user() to update guest PTE A/D bits
  KVM: x86: Use __try_cmpxchg_user() to emulate atomic accesses
  KVM: x86: Bail to userspace if emulation of atomic user access faults

 arch/x86/include/asm/uaccess.h | 131 +++++++++++++++++++++++++++++++++
 arch/x86/kvm/mmu/paging_tmpl.h |  45 +----------
 arch/x86/kvm/x86.c             |  35 ++++-----
 init/Kconfig                   |   4 +
 4 files changed, 150 insertions(+), 65 deletions(-)


base-commit: 26291c54e111ff6ba87a164d85d4a4e134b7315c
-- 
2.35.0.rc2.247.g8bbb082509-goog

[PATCH 1/5] Kconfig: Add option for asm goto w/ tied outputs to workaround clang-13 bug

[Sean Christopherson]

Add a config option to guard (future) usage of asm_volatile_goto() that
includes "tied outputs", i.e. "+" constraints that specify both an input
and output parameter.  clang-13 has a bug[1] that causes compilation of
such inline asm to fail, and KVM wants to use a "+m" constraint to
implement a uaccess form of CMPXCHG[2].  E.g. the test code fails with

  <stdin>:1:29: error: invalid operand in inline asm: '.long (${1:l}) - .'
  int foo(int *x) { asm goto (".long (%l[bar]) - .\n": "+m"(*x) ::: bar); return *x; bar: return 0; }
                            ^
  <stdin>:1:29: error: unknown token in expression
  <inline asm>:1:9: note: instantiated into assembly here
          .long () - .
                 ^
  2 errors generated.

on clang-13, but passes on gcc (with appropriate asm goto support).  The
bug is fixed in clang-14, but won't be backported to clang-13 as the
changes are too invasive/risky.

[1] https://github.com/ClangBuiltLinux/linux/issues/1512
[2] https://lore.kernel.org/all/YfMruK8%2F1izZ2VHS@google.com

Suggested-by: Nick Desaulniers <ndesaulniers@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 init/Kconfig | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/init/Kconfig b/init/Kconfig
index e9119bf54b1f..a206b21703be 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -77,6 +77,10 @@ config CC_HAS_ASM_GOTO_OUTPUT
 	depends on CC_HAS_ASM_GOTO
 	def_bool $(success,echo 'int foo(int x) { asm goto ("": "=r"(x) ::: bar); return x; bar: return 0; }' | $(CC) -x c - -c -o /dev/null)
 
+config CC_HAS_ASM_GOTO_TIED_OUTPUT
+	depends on CC_HAS_ASM_GOTO_OUTPUT
+	def_bool $(success,echo 'int foo(int *x) { asm goto (".long (%l[bar]) - .\n": "+m"(*x) ::: bar); return *x; bar: return 0; }' | $CC -x c - -c -o /dev/null)
+
 config TOOLS_SUPPORT_RELR
 	def_bool $(success,env "CC=$(CC)" "LD=$(LD)" "NM=$(NM)" "OBJCOPY=$(OBJCOPY)" $(srctree)/scripts/tools-support-relr.sh)
 
-- 
2.35.0.rc2.247.g8bbb082509-goog

[PATCH 2/5] x86/uaccess: Implement macros for CMPXCHG on user addresses

[Sean Christopherson]

From: Peter Zijlstra <peterz@infradead.org>

Add support for CMPXCHG loops on userspace addresses.  Provide both an
"unsafe" version for tight loops that do their own uaccess begin/end, as
well as a "safe" version for use cases where the CMPXCHG is not buried in
a loop, e.g. KVM will resume the guest instead of looping when emulation
of a guest atomic accesses fails the CMPXCHG.

Provide 8-byte versions for 32-bit kernels so that KVM can do CMPXCHG on
guest PAE PTEs, which are accessed via userspace addresses.

Guard the asm_volatile_goto() variation with CC_HAS_ASM_GOTO_TIED_OUTPUT,
the "+m" constraint fails on some compilers that otherwise support
CC_HAS_ASM_GOTO_OUTPUT.

Cc: stable@vger.kernel.org
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Co-developed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/include/asm/uaccess.h | 131 +++++++++++++++++++++++++++++++++
 1 file changed, 131 insertions(+)

diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
index ac96f9b2d64b..423bfcc1ec4b 100644
--- a/arch/x86/include/asm/uaccess.h
+++ b/arch/x86/include/asm/uaccess.h
@@ -409,6 +409,98 @@ do {									\
 
 #endif // CONFIG_CC_HAS_ASM_GOTO_OUTPUT
 
+#ifdef CONFIG_CC_HAS_ASM_GOTO_TIED_OUTPUT
+#define __try_cmpxchg_user_asm(itype, ltype, _ptr, _pold, _new, label)	({ \
+	bool success;							\
+	__typeof__(_ptr) _old = (__typeof__(_ptr))(_pold);		\
+	__typeof__(*(_ptr)) __old = *_old;				\
+	__typeof__(*(_ptr)) __new = (_new);				\
+	asm_volatile_goto("\n"						\
+		     "1: " LOCK_PREFIX "cmpxchg"itype" %[new], %[ptr]\n"\
+		     _ASM_EXTABLE_UA(1b, %l[label])			\
+		     : CC_OUT(z) (success),				\
+		       [ptr] "+m" (*_ptr),				\
+		       [old] "+a" (__old)				\
+		     : [new] ltype (__new)				\
+		     : "memory"						\
+		     : label);						\
+	if (unlikely(!success))						\
+		*_old = __old;						\
+	likely(success);					})
+
+#ifdef CONFIG_X86_32
+#define __try_cmpxchg64_user_asm(_ptr, _pold, _new, label)	({	\
+	bool success;							\
+	__typeof__(_ptr) _old = (__typeof__(_ptr))(_pold);		\
+	__typeof__(*(_ptr)) __old = *_old;				\
+	__typeof__(*(_ptr)) __new = (_new);				\
+	asm_volatile_goto("\n"						\
+		     "1: " LOCK_PREFIX "cmpxchg8b %[ptr]\n"		\
+		     _ASM_EXTABLE_UA(1b, %l[label])			\
+		     : CC_OUT(z) (success),				\
+		       "+A" (__old),					\
+		       [ptr] "+m" (*_ptr)				\
+		     : "b" ((u32)__new),				\
+		       "c" ((u32)((u64)__new >> 32))			\
+		     : "memory"						\
+		     : label);						\
+	if (unlikely(!success))						\
+		*_old = __old;						\
+	likely(success);					})
+#endif // CONFIG_X86_32
+#else  // !CONFIG_CC_HAS_ASM_GOTO_TIED_OUTPUT
+#define __try_cmpxchg_user_asm(itype, ltype, _ptr, _pold, _new, label)	({ \
+	int __err = 0;							\
+	bool success;							\
+	__typeof__(_ptr) _old = (__typeof__(_ptr))(_pold);		\
+	__typeof__(*(_ptr)) __old = *_old;				\
+	__typeof__(*(_ptr)) __new = (_new);				\
+	asm volatile("\n"						\
+		     "1: " LOCK_PREFIX "cmpxchg"itype" %[new], %[ptr]\n"\
+		     CC_SET(z)						\
+		     "2:\n"						\
+		     _ASM_EXTABLE_TYPE_REG(1b, 2b, EX_TYPE_EFAULT_REG,	\
+					   %[errout])			\
+		     : CC_OUT(z) (success),				\
+		       [errout] "+r" (__err),				\
+		       [ptr] "+m" (*_ptr),				\
+		       [old] "+a" (__old)				\
+		     : [new] ltype (__new)				\
+		     : "memory", "cc");					\
+	if (unlikely(__err))						\
+		goto label;						\
+	if (unlikely(!success))						\
+		*_old = __old;						\
+	likely(success);					})
+
+#ifdef CONFIG_X86_32
+#define __try_cmpxchg64_user_asm(_ptr, _pold, _new, label)	({	\
+	int __err = 0;							\
+	bool success;							\
+	__typeof__(_ptr) _old = (__typeof__(_ptr))(_pold);		\
+	__typeof__(*(_ptr)) __old = *_old;				\
+	__typeof__(*(_ptr)) __new = (_new);				\
+	asm volatile("\n"						\
+		     "1: " LOCK_PREFIX "cmpxchg8b %[ptr]\n"		\
+		     CC_SET(z)						\
+		     "2:\n"						\
+		     _ASM_EXTABLE_TYPE_REG(1b, 2b, EX_TYPE_EFAULT_REG,	\
+					   %[errout])			\
+		     : CC_OUT(z) (success),				\
+		       [errout] "+r" (__err),				\
+		       "+A" (__old),					\
+		       [ptr] "+m" (*_ptr)				\
+		     : "b" ((u32)__new),				\
+		       "c" ((u32)((u64)__new >> 32))			\
+		     : "memory", "cc");					\
+	if (unlikely(__err))						\
+		goto label;						\
+	if (unlikely(!success))						\
+		*_old = __old;						\
+	likely(success);					})
+#endif // CONFIG_X86_32
+#endif // CONFIG_CC_HAS_ASM_GOTO_TIED_OUTPUT
+
 /* FIXME: this hack is definitely wrong -AK */
 struct __large_struct { unsigned long buf[100]; };
 #define __m(x) (*(struct __large_struct __user *)(x))
@@ -501,6 +593,45 @@ do {										\
 } while (0)
 #endif // CONFIG_CC_HAS_ASM_GOTO_OUTPUT
 
+extern void __try_cmpxchg_user_wrong_size(void);
+
+#ifndef CONFIG_X86_32
+#define __try_cmpxchg64_user_asm(_ptr, _oldp, _nval, _label)		\
+	__try_cmpxchg_user_asm("q", "r", (_ptr), (_oldp), (_nval), _label)
+#endif
+
+#define unsafe_try_cmpxchg_user(_ptr, _oldp, _nval, _label) ({		\
+	bool __ret;							\
+	switch (sizeof(*(_ptr))) {					\
+	case 1:	__ret = __try_cmpxchg_user_asm("b", "q",		\
+					       (_ptr), (_oldp),		\
+					       (_nval), _label);	\
+		break;							\
+	case 2:	__ret = __try_cmpxchg_user_asm("w", "r",		\
+					       (_ptr), (_oldp),		\
+					       (_nval), _label);	\
+		break;							\
+	case 4:	__ret = __try_cmpxchg_user_asm("l", "r",		\
+					       (_ptr), (_oldp),		\
+					       (_nval), _label);	\
+		break;							\
+	case 8:	__ret = __try_cmpxchg64_user_asm((_ptr), (_oldp),	\
+						 (_nval), _label);	\
+		break;							\
+	default: __try_cmpxchg_user_wrong_size();			\
+	}								\
+	__ret;						})
+
+/* "Returns" 0 on success, 1 on failure, -EFAULT if the access faults. */
+#define __try_cmpxchg_user(_ptr, _oldp, _nval, _label)	({		\
+	int __ret = -EFAULT;						\
+	__uaccess_begin_nospec();					\
+	__ret = !unsafe_try_cmpxchg_user(_ptr, _oldp, _nval, _label);	\
+_label:									\
+	__uaccess_end();						\
+	__ret;								\
+							})
+
 /*
  * We want the unsafe accessors to always be inlined and use
  * the error labels - thus the macro games.
-- 
2.35.0.rc2.247.g8bbb082509-goog

[PATCH 3/5] KVM: x86: Use __try_cmpxchg_user() to update guest PTE A/D bits

[Sean Christopherson]

Use the recently introduced __try_cmpxchg_user() to update guest PTE A/D
bits instead of mapping the PTE into kernel address space.  The VM_PFNMAP
path is broken as it assumes that vm_pgoff is the base pfn of the mapped
VMA range, which is conceptually wrong as vm_pgoff is the offset relative
to the file and has nothing to do with the pfn.  The horrific hack worked
for the original use case (backing guest memory with /dev/mem), but leads
to accessing "random" pfns for pretty much any other VM_PFNMAP case.

Fixes: bd53cb35a3e9 ("X86/KVM: Handle PFNs outside of kernel reach when touching GPTEs")
Debugged-by: Tadeusz Struk <tadeusz.struk@linaro.org>
Reported-by: syzbot+6cde2282daa792c49ab8@syzkaller.appspotmail.com
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/mmu/paging_tmpl.h | 45 +---------------------------------
 1 file changed, 1 insertion(+), 44 deletions(-)

diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
index 5b5bdac97c7b..551de15f342f 100644
--- a/arch/x86/kvm/mmu/paging_tmpl.h
+++ b/arch/x86/kvm/mmu/paging_tmpl.h
@@ -143,49 +143,6 @@ static bool FNAME(is_rsvd_bits_set)(struct kvm_mmu *mmu, u64 gpte, int level)
 	       FNAME(is_bad_mt_xwr)(&mmu->guest_rsvd_check, gpte);
 }
 
-static int FNAME(cmpxchg_gpte)(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu,
-			       pt_element_t __user *ptep_user, unsigned index,
-			       pt_element_t orig_pte, pt_element_t new_pte)
-{
-	int npages;
-	pt_element_t ret;
-	pt_element_t *table;
-	struct page *page;
-
-	npages = get_user_pages_fast((unsigned long)ptep_user, 1, FOLL_WRITE, &page);
-	if (likely(npages == 1)) {
-		table = kmap_atomic(page);
-		ret = CMPXCHG(&table[index], orig_pte, new_pte);
-		kunmap_atomic(table);
-
-		kvm_release_page_dirty(page);
-	} else {
-		struct vm_area_struct *vma;
-		unsigned long vaddr = (unsigned long)ptep_user & PAGE_MASK;
-		unsigned long pfn;
-		unsigned long paddr;
-
-		mmap_read_lock(current->mm);
-		vma = find_vma_intersection(current->mm, vaddr, vaddr + PAGE_SIZE);
-		if (!vma || !(vma->vm_flags & VM_PFNMAP)) {
-			mmap_read_unlock(current->mm);
-			return -EFAULT;
-		}
-		pfn = ((vaddr - vma->vm_start) >> PAGE_SHIFT) + vma->vm_pgoff;
-		paddr = pfn << PAGE_SHIFT;
-		table = memremap(paddr, PAGE_SIZE, MEMREMAP_WB);
-		if (!table) {
-			mmap_read_unlock(current->mm);
-			return -EFAULT;
-		}
-		ret = CMPXCHG(&table[index], orig_pte, new_pte);
-		memunmap(table);
-		mmap_read_unlock(current->mm);
-	}
-
-	return (ret != orig_pte);
-}
-
 static bool FNAME(prefetch_invalid_gpte)(struct kvm_vcpu *vcpu,
 				  struct kvm_mmu_page *sp, u64 *spte,
 				  u64 gpte)
@@ -284,7 +241,7 @@ static int FNAME(update_accessed_dirty_bits)(struct kvm_vcpu *vcpu,
 		if (unlikely(!walker->pte_writable[level - 1]))
 			continue;
 
-		ret = FNAME(cmpxchg_gpte)(vcpu, mmu, ptep_user, index, orig_pte, pte);
+		ret = __try_cmpxchg_user(ptep_user, &orig_pte, pte, fault);
 		if (ret)
 			return ret;
 
-- 
2.35.0.rc2.247.g8bbb082509-goog

[PATCH 4/5] KVM: x86: Use __try_cmpxchg_user() to emulate atomic accesses

[Sean Christopherson]

Use the recently introduce __try_cmpxchg_user() to emulate atomic guest
accesses via the associated userspace address instead of mapping the
backing pfn into kernel address space.  Using kvm_vcpu_map() is unsafe as
it does not coordinate with KVM's mmu_notifier to ensure the hva=>pfn
translation isn't changed/unmapped in the memremap() path, i.e. when
there's no struct page and thus no elevated refcount.

Fixes: 42e35f8072c3 ("KVM/X86: Use kvm_vcpu_map in emulator_cmpxchg_emulated")
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/x86.c | 35 ++++++++++++++---------------------
 1 file changed, 14 insertions(+), 21 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 74b53a16f38a..37064d565bbc 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -7155,15 +7155,8 @@ static int emulator_write_emulated(struct x86_emulate_ctxt *ctxt,
 				   exception, &write_emultor);
 }
 
-#define CMPXCHG_TYPE(t, ptr, old, new) \
-	(cmpxchg((t *)(ptr), *(t *)(old), *(t *)(new)) == *(t *)(old))
-
-#ifdef CONFIG_X86_64
-#  define CMPXCHG64(ptr, old, new) CMPXCHG_TYPE(u64, ptr, old, new)
-#else
-#  define CMPXCHG64(ptr, old, new) \
-	(cmpxchg64((u64 *)(ptr), *(u64 *)(old), *(u64 *)(new)) == *(u64 *)(old))
-#endif
+#define emulator_try_cmpxchg_user(t, ptr, old, new) \
+	(__try_cmpxchg_user((t *)(ptr), (t *)(old), *(t *)(new), efault ## t))
 
 static int emulator_cmpxchg_emulated(struct x86_emulate_ctxt *ctxt,
 				     unsigned long addr,
@@ -7172,12 +7165,11 @@ static int emulator_cmpxchg_emulated(struct x86_emulate_ctxt *ctxt,
 				     unsigned int bytes,
 				     struct x86_exception *exception)
 {
-	struct kvm_host_map map;
 	struct kvm_vcpu *vcpu = emul_to_vcpu(ctxt);
 	u64 page_line_mask;
+	unsigned long hva;
 	gpa_t gpa;
-	char *kaddr;
-	bool exchanged;
+	int r;
 
 	/* guests cmpxchg8b have to be emulated atomically */
 	if (bytes > 8 || (bytes & (bytes - 1)))
@@ -7201,31 +7193,32 @@ static int emulator_cmpxchg_emulated(struct x86_emulate_ctxt *ctxt,
 	if (((gpa + bytes - 1) & page_line_mask) != (gpa & page_line_mask))
 		goto emul_write;
 
-	if (kvm_vcpu_map(vcpu, gpa_to_gfn(gpa), &map))
+	hva = kvm_vcpu_gfn_to_hva(vcpu, gpa_to_gfn(gpa));
+	if (kvm_is_error_hva(addr))
 		goto emul_write;
 
-	kaddr = map.hva + offset_in_page(gpa);
+	hva += offset_in_page(gpa);
 
 	switch (bytes) {
 	case 1:
-		exchanged = CMPXCHG_TYPE(u8, kaddr, old, new);
+		r = emulator_try_cmpxchg_user(u8, hva, old, new);
 		break;
 	case 2:
-		exchanged = CMPXCHG_TYPE(u16, kaddr, old, new);
+		r = emulator_try_cmpxchg_user(u16, hva, old, new);
 		break;
 	case 4:
-		exchanged = CMPXCHG_TYPE(u32, kaddr, old, new);
+		r = emulator_try_cmpxchg_user(u32, hva, old, new);
 		break;
 	case 8:
-		exchanged = CMPXCHG64(kaddr, old, new);
+		r = emulator_try_cmpxchg_user(u64, hva, old, new);
 		break;
 	default:
 		BUG();
 	}
 
-	kvm_vcpu_unmap(vcpu, &map, true);
-
-	if (!exchanged)
+	if (r < 0)
+		goto emul_write;
+	if (r)
 		return X86EMUL_CMPXCHG_FAILED;
 
 	kvm_page_track_write(vcpu, gpa, new, bytes);
-- 
2.35.0.rc2.247.g8bbb082509-goog

[PATCH 5/5] KVM: x86: Bail to userspace if emulation of atomic user access faults

[Sean Christopherson]

Exit to userspace when emulating an atomic guest access if the CMPXCHG on
the userspace address faults.  Emulating the access as a write and thus
likely treating it as emulated MMIO is wrong, as KVM has already
confirmed there is a valid, writable memslot.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/x86.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 37064d565bbc..66c5410dd4c3 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -7217,7 +7217,7 @@ static int emulator_cmpxchg_emulated(struct x86_emulate_ctxt *ctxt,
 	}
 
 	if (r < 0)
-		goto emul_write;
+		return X86EMUL_UNHANDLEABLE;
 	if (r)
 		return X86EMUL_CMPXCHG_FAILED;
 
-- 
2.35.0.rc2.247.g8bbb082509-goog

Re: [PATCH 0/5] x86: uaccess CMPXCHG + KVM bug fixes

[Tadeusz Struk]

On 1/31/22 17:08, Sean Christopherson wrote:
> Add uaccess macros for doing CMPXCHG on userspace addresses and use the
> macros to fix KVM bugs by replacing flawed code that maps memory into the
> kernel address space without proper mmu_notifier protection (or with
> broken pfn calculations in one case).
> 
> Add yet another Kconfig for guarding asm_volatile_goto() to workaround a
> clang-13 bug.  I've verified the test passes on gcc versions of arm64,
> PPC, RISC-V, and s390x that also pass the CC_HAS_ASM_GOTO_OUTPUT test.
> 
> Patches 1-4 are tagged for stable@ as patches 3 and 4 (mostly 3) need a
> backportable fix, and doing CMPXCHG on the userspace address is the
> simplest fix from a KVM perspective.
> 
> Peter Zijlstra (1):
>    x86/uaccess: Implement macros for CMPXCHG on user addresses
> 
> Sean Christopherson (4):
>    Kconfig: Add option for asm goto w/ tied outputs to workaround
>      clang-13 bug
>    KVM: x86: Use __try_cmpxchg_user() to update guest PTE A/D bits
>    KVM: x86: Use __try_cmpxchg_user() to emulate atomic accesses
>    KVM: x86: Bail to userspace if emulation of atomic user access faults
> 
>   arch/x86/include/asm/uaccess.h | 131 +++++++++++++++++++++++++++++++++
>   arch/x86/kvm/mmu/paging_tmpl.h |  45 +----------
>   arch/x86/kvm/x86.c             |  35 ++++-----
>   init/Kconfig                   |   4 +
>   4 files changed, 150 insertions(+), 65 deletions(-)

This also fixes the following syzbot issue:
https://syzkaller.appspot.com/bug?id=6cb6102a0a7b0c52060753dd62d070a1d1e71347

Tested-by: Tadeusz Struk <tadeusz.struk@linaro.org>

--
Thanks,
Tadeusz

Re: [PATCH 1/5] Kconfig: Add option for asm goto w/ tied outputs to workaround clang-13 bug

[Nick Desaulniers]

On Mon, Jan 31, 2022 at 5:08 PM Sean Christopherson <seanjc@google.com> wrote:
>
> Add a config option to guard (future) usage of asm_volatile_goto() that
> includes "tied outputs", i.e. "+" constraints that specify both an input
> and output parameter.  clang-13 has a bug[1] that causes compilation of
> such inline asm to fail, and KVM wants to use a "+m" constraint to
> implement a uaccess form of CMPXCHG[2].  E.g. the test code fails with
>
>   <stdin>:1:29: error: invalid operand in inline asm: '.long (${1:l}) - .'
>   int foo(int *x) { asm goto (".long (%l[bar]) - .\n": "+m"(*x) ::: bar); return *x; bar: return 0; }
>                             ^
>   <stdin>:1:29: error: unknown token in expression
>   <inline asm>:1:9: note: instantiated into assembly here
>           .long () - .
>                  ^
>   2 errors generated.
>
> on clang-13, but passes on gcc (with appropriate asm goto support).  The
> bug is fixed in clang-14, but won't be backported to clang-13 as the
> changes are too invasive/risky.

LGTM.
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

If you're going to respin the series, consider adding a comment in the
source along the lines of:
```
clang-14 and gcc-11 fixed this.
```
or w/e. This helps us find (via grep) and remove cruft when the
minimum supported compiler versions are updated.

Note: gcc-10 had a bug with the symbolic references to labels when
using tied constraints.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98096

Both compilers had bugs here, and it may be worth mentioning that in
the commit message.

>
> [1] https://github.com/ClangBuiltLinux/linux/issues/1512
> [2] https://lore.kernel.org/all/YfMruK8%2F1izZ2VHS@google.com
>
> Suggested-by: Nick Desaulniers <ndesaulniers@google.com>
> Cc: stable@vger.kernel.org
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>  init/Kconfig | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/init/Kconfig b/init/Kconfig
> index e9119bf54b1f..a206b21703be 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -77,6 +77,10 @@ config CC_HAS_ASM_GOTO_OUTPUT
>         depends on CC_HAS_ASM_GOTO
>         def_bool $(success,echo 'int foo(int x) { asm goto ("": "=r"(x) ::: bar); return x; bar: return 0; }' | $(CC) -x c - -c -o /dev/null)
>
> +config CC_HAS_ASM_GOTO_TIED_OUTPUT
> +       depends on CC_HAS_ASM_GOTO_OUTPUT
> +       def_bool $(success,echo 'int foo(int *x) { asm goto (".long (%l[bar]) - .\n": "+m"(*x) ::: bar); return *x; bar: return 0; }' | $CC -x c - -c -o /dev/null)
> +
>  config TOOLS_SUPPORT_RELR
>         def_bool $(success,env "CC=$(CC)" "LD=$(LD)" "NM=$(NM)" "OBJCOPY=$(OBJCOPY)" $(srctree)/scripts/tools-support-relr.sh)
>
> --
> 2.35.0.rc2.247.g8bbb082509-goog
>


-- 
Thanks,
~Nick Desaulniers

Re: [PATCH 1/5] Kconfig: Add option for asm goto w/ tied outputs to workaround clang-13 bug

[Sean Christopherson]

On Tue, Feb 01, 2022, Nick Desaulniers wrote:
> On Mon, Jan 31, 2022 at 5:08 PM Sean Christopherson <seanjc@google.com> wrote:
> >
> > Add a config option to guard (future) usage of asm_volatile_goto() that
> > includes "tied outputs", i.e. "+" constraints that specify both an input
> > and output parameter.  clang-13 has a bug[1] that causes compilation of
> > such inline asm to fail, and KVM wants to use a "+m" constraint to
> > implement a uaccess form of CMPXCHG[2].  E.g. the test code fails with
> >
> >   <stdin>:1:29: error: invalid operand in inline asm: '.long (${1:l}) - .'
> >   int foo(int *x) { asm goto (".long (%l[bar]) - .\n": "+m"(*x) ::: bar); return *x; bar: return 0; }
> >                             ^
> >   <stdin>:1:29: error: unknown token in expression
> >   <inline asm>:1:9: note: instantiated into assembly here
> >           .long () - .
> >                  ^
> >   2 errors generated.
> >
> > on clang-13, but passes on gcc (with appropriate asm goto support).  The
> > bug is fixed in clang-14, but won't be backported to clang-13 as the
> > changes are too invasive/risky.
> 
> LGTM.
> Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
> 
> If you're going to respin the series, consider adding a comment in the
> source along the lines of:
> ```
> clang-14 and gcc-11 fixed this.
> ```
> or w/e. This helps us find (via grep) and remove cruft when the
> minimum supported compiler versions are updated.

Will do, a new version is definitely needed.

> Note: gcc-10 had a bug with the symbolic references to labels when
> using tied constraints.
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98096
> 
> Both compilers had bugs here, and it may be worth mentioning that in
> the commit message.

Is this wording accurate?

  gcc also had a similar bug[3], fixed in gcc-11, where gcc failed to
  account for its behavior of assigning two numbers to tied outputs (one
  for input, one for output) when evaluating symbolic references. 

Re: [PATCH 1/5] Kconfig: Add option for asm goto w/ tied outputs to workaround clang-13 bug

[Nick Desaulniers]

On Tue, Feb 1, 2022 at 12:56 PM Sean Christopherson <seanjc@google.com> wrote:
>
> On Tue, Feb 01, 2022, Nick Desaulniers wrote:
> > Note: gcc-10 had a bug with the symbolic references to labels when
> > using tied constraints.
> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98096
> >
> > Both compilers had bugs here, and it may be worth mentioning that in
> > the commit message.
>
> Is this wording accurate?
>
>   gcc also had a similar bug[3], fixed in gcc-11, where gcc failed to
>   account for its behavior of assigning two numbers to tied outputs (one
>   for input, one for output) when evaluating symbolic references.

SGTM
-- 
Thanks,
~Nick Desaulniers