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

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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@arm.com>
Acked-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: James Morse <james.morse@arm.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20211019160219.5202-4-mark.rutland@arm.com
Signed-off-by: Will Deacon <will@kernel.org>
This commit is contained in:
Mark Rutland 2021-10-19 17:02:09 +01:00 committed by Will Deacon
parent 4012e0e227
commit 139f9ab73d
1 changed files with 3 additions and 5 deletions
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8
arch/arm64/lib/copy_to_user.S
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@ -59,11 +59,8 @@ 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
// Exception fixups
9997: cmp dst, dstin
b.ne 9998f
// Before being absolutely sure we couldn't copy anything, try harder
@ -72,4 +69,5 @@ 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)