[PATCH 03/13] arm64: lib: __arch_copy_to_user(): fold fixups into body

[Mark Rutland]

Like other functions, __arch_copy_to_user() places its exception fixups
in the `.fixup` section without any clear association with
__arch_copy_to_user() itself. If we backtrace the fixup code, it will be
symbolized as an offset from the nearest prior symbol, which happens to
be `__entry_tramp_text_end`. Further, since the PC adjustment for the
fixup is akin to a direct branch rather than a function call,
__arch_copy_to_user() itself will be missing from the backtrace.

This is confusing and hinders debugging. In general this pattern will
also be problematic for CONFIG_LIVEPATCH, since fixups often return to
their associated function, but this isn't accurately captured in the
stacktrace.

To solve these issues for assembly functions, we must move fixups into
the body of the functions themselves, after the usual fast-path returns.
This patch does so for __arch_copy_to_user().

Inline assembly will be dealt with in subsequent patches.

Other than the improved backtracing, there should be no functional
change as a result of this patch.

Signed-off-by: Mark Rutland <mark.rutland at arm.com>
Cc: Ard Biesheuvel <ardb at kernel.org>
Cc: Catalin Marinas <catalin.marinas at arm.com>
Cc: James Morse <james.morse at arm.com>
Cc: Mark Brown <broonie at kernel.org>
Cc: Robin Murphy <robin.murphy at arm.com>
Cc: Will Deacon <will at kernel.org>
---
 arch/arm64/lib/copy_to_user.S | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/arch/arm64/lib/copy_to_user.S b/arch/arm64/lib/copy_to_user.S
index 9f380eecf653..2afc27bc0681 100644
--- a/arch/arm64/lib/copy_to_user.S
+++ b/arch/arm64/lib/copy_to_user.S
@@ -59,11 +59,7 @@ SYM_FUNC_START(__arch_copy_to_user)
 #include "copy_template.S"
 	mov	x0, #0
 	ret
-SYM_FUNC_END(__arch_copy_to_user)
-EXPORT_SYMBOL(__arch_copy_to_user)
 
-	.section .fixup,"ax"
-	.align	2
 9997:	cmp	dst, dstin
 	b.ne	9998f
 	// Before being absolutely sure we couldn't copy anything, try harder
@@ -72,4 +68,5 @@ USER(9998f, sttrb tmp1w, [dst])
 	add	dst, dst, #1
 9998:	sub	x0, end, dst			// bytes not copied
 	ret
-	.previous
+SYM_FUNC_END(__arch_copy_to_user)
+EXPORT_SYMBOL(__arch_copy_to_user)
-- 
2.11.0