arm64: lib: __arch_clear_user(): fold fixups into body

Like other functions, __arch_clear_user() places its exception fixups in
the `.fixup` section without any clear association with
__arch_clear_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_clear_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_clear_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 <[email protected]>
Acked-by: Robin Murphy <[email protected]>
Reviewed-by: Ard Biesheuvel <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: James Morse <[email protected]>
Cc: Mark Brown <[email protected]>
Cc: Will Deacon <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Will Deacon <[email protected]>

Author: mrutland-arm

arch/arm64/lib/clear_user.S

@@ -45,13 +45,11 @@ USER(9f, sttrh	wzr, [x0])
 USER(7f, sttrb	wzr, [x2, #-1])
 5:	mov	x0, #0
 	ret
-SYM_FUNC_END(__arch_clear_user)
-EXPORT_SYMBOL(__arch_clear_user)
 
-	.section .fixup,"ax"
-	.align	2
+	// Exception fixups
 7:	sub	x0, x2, #5	// Adjust for faulting on the final byte...
 8:	add	x0, x0, #4	// ...or the second word of the 4-7 byte case
 9:	sub	x0, x2, x0
 	ret
-	.previous
+SYM_FUNC_END(__arch_clear_user)
+EXPORT_SYMBOL(__arch_clear_user)