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Date:   Fri, 1 Mar 2019 13:57:18 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>, Peter Anvin <hpa@zytor.com>,
        Julien Thierry <julien.thierry@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Ingo Molnar <mingo@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        James Morse <james.morse@arm.com>, valentin.schneider@arm.com,
        Brian Gerst <brgerst@gmail.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Andrew Lutomirski <luto@kernel.org>,
        Borislav Petkov <bp@alien8.de>,
        Denys Vlasenko <dvlasenk@redhat.com>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Chris Wilson <chris@chris-wilson.co.uk>
Subject: Re: [PATCH 6/8] i915,uaccess: Fix redundant CLAC
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On Fri, Mar 01, 2019 at 01:27:45PM +0100, Peter Zijlstra wrote:
> arch/x86/lib/usercopy_64.o: warning: objtool: .altinstr_replacement+0x30: redundant UACCESS disable

> The usercopy one is difficult, that's copy_user_handle_tail(), it is
> buggered though, because that lacks notrace and thus has a __fentry__
> call in.
> 
> Also, afaict all exception jumps into copy_user_handle_tail() will have
> AC=1, but the __{get,put}_user_nocheck() things do STAC/CLAC all over
> again.
> 
> So what do we do? Annotate that we start with AC=1 and then immediately
> do the clac, and then let __{get,put}_user_nocheck() do their own thing?
> or make it use the unsafe stuff?

Or.. we move the thing to assembly. Of course, I suck at (writing) asm,
so the below is probably broken in various ways.

--- a/arch/x86/include/asm/uaccess_64.h
+++ b/arch/x86/include/asm/uaccess_64.h
@@ -208,9 +208,6 @@ __copy_from_user_flushcache(void *dst, c
 }
 
 unsigned long
-copy_user_handle_tail(char *to, char *from, unsigned len);
-
-unsigned long
 mcsafe_handle_tail(char *to, char *from, unsigned len);
 
 #endif /* _ASM_X86_UACCESS_64_H */
--- a/arch/x86/lib/copy_user_64.S
+++ b/arch/x86/lib/copy_user_64.S
@@ -194,6 +194,29 @@ ENDPROC(copy_user_enhanced_fast_string)
 EXPORT_SYMBOL(copy_user_enhanced_fast_string)
 
 /*
+ * Try to copy last bytes and clear the rest if needed.
+ * Since protection fault in copy_from/to_user is not a normal situation,
+ * it is not necessary to optimize tail handling.
+ *
+ * Input:
+ * rdi destination
+ * rsi source
+ * rdx count
+ *
+ * Output:
+ * eax uncopied bytes or 0 if successful.
+ */
+ENTRY(copy_user_handle_tail)
+	movl %edx,%ecx
+1:	rep movsb
+2:	mov %ecx,%eax
+	ASM_CLAC
+	ret
+
+	_ASM_EXTABLE_UA(1b, 2b)
+END(copy_user_handle_tail)
+
+/*
  * copy_user_nocache - Uncached memory copy with exception handling
  * This will force destination out of cache for more performance.
  *
--- a/arch/x86/lib/usercopy_64.c
+++ b/arch/x86/lib/usercopy_64.c
@@ -55,26 +55,6 @@ unsigned long clear_user(void __user *to
 EXPORT_SYMBOL(clear_user);
 
 /*
- * Try to copy last bytes and clear the rest if needed.
- * Since protection fault in copy_from/to_user is not a normal situation,
- * it is not necessary to optimize tail handling.
- */
-__visible unsigned long
-copy_user_handle_tail(char *to, char *from, unsigned len)
-{
-	for (; len; --len, to++) {
-		char c;
-
-		if (__get_user_nocheck(c, from++, sizeof(char)))
-			break;
-		if (__put_user_nocheck(c, to, sizeof(char)))
-			break;
-	}
-	clac();
-	return len;
-}
-
-/*
  * Similar to copy_user_handle_tail, probe for the write fault point,
  * but reuse __memcpy_mcsafe in case a new read error is encountered.
  * clac() is handled in _copy_to_iter_mcsafe().