[PATCH] arm64: ftrace: forbid CALL_OPS with CC_OPTIMIZE_FOR_SIZE

[PATCH] arm64: ftrace: forbid CALL_OPS with CC_OPTIMIZE_FOR_SIZE

[Mark Rutland]

Florian reports that when building with CONFIG_CC_OPTIMIZE_FOR_SIZE=y,
he sees "Misaligned patch-site" warnings at boot, e.g.

| Misaligned patch-site bcm2836_arm_irqchip_handle_irq+0x0/0x88
| WARNING: CPU: 0 PID: 0 at arch/arm64/kernel/ftrace.c:120 ftrace_call_adjust+0x4c/0x70

This is because GCC will silently ignore `-falign-functions=N` when
passed `-Os`, resulting in functions not being aligned as we expect.
This is a known issue, and to account for this we modified the kernel to
avoid `-Os` generally. Unfortunately we forgot to account for
CONFIG_CC_OPTIMIZE_FOR_SIZE.

Forbid the use of CALL_OPS with CONFIG_CC_OPTIMIZE_FOR_SIZE=y to prevent
this issue. All exising ftrace features will work as before, though
without the performance benefit of CALL_OPS.

Reported-by: Florian Fainelli <f.fainelli@gmail.com>
Link: http://lore.kernel.org/linux-arm-kernel/2d9284c3-3805-402b-5423-520ced56d047@gmail.com
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Stefan Wahren <stefan.wahren@i2se.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Will Deacon <will@kernel.org>
---
 arch/arm64/Kconfig | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index 27b2592698b0..318c61730d44 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -187,7 +187,8 @@ config ARM64
 	select HAVE_DMA_CONTIGUOUS
 	select HAVE_DYNAMIC_FTRACE
 	select HAVE_DYNAMIC_FTRACE_WITH_CALL_OPS \
-		if (DYNAMIC_FTRACE_WITH_ARGS && !CFI_CLANG)
+		if (DYNAMIC_FTRACE_WITH_ARGS && !CFI_CLANG && \
+		    !CC_OPTIMIZE_FOR_SIZE)
 	select FTRACE_MCOUNT_USE_PATCHABLE_FUNCTION_ENTRY \
 		if DYNAMIC_FTRACE_WITH_ARGS
 	select HAVE_EFFICIENT_UNALIGNED_ACCESS
-- 
2.30.2


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Re: [PATCH] arm64: ftrace: forbid CALL_OPS with CC_OPTIMIZE_FOR_SIZE

[Florian Fainelli]

On 2/27/2023 3:58 AM, Mark Rutland wrote:
> Florian reports that when building with CONFIG_CC_OPTIMIZE_FOR_SIZE=y,
> he sees "Misaligned patch-site" warnings at boot, e.g.
> 
> | Misaligned patch-site bcm2836_arm_irqchip_handle_irq+0x0/0x88
> | WARNING: CPU: 0 PID: 0 at arch/arm64/kernel/ftrace.c:120 ftrace_call_adjust+0x4c/0x70
> 
> This is because GCC will silently ignore `-falign-functions=N` when
> passed `-Os`, resulting in functions not being aligned as we expect.
> This is a known issue, and to account for this we modified the kernel to
> avoid `-Os` generally. Unfortunately we forgot to account for
> CONFIG_CC_OPTIMIZE_FOR_SIZE.
> 
> Forbid the use of CALL_OPS with CONFIG_CC_OPTIMIZE_FOR_SIZE=y to prevent
> this issue. All exising ftrace features will work as before, though
> without the performance benefit of CALL_OPS.
> 
> Reported-by: Florian Fainelli <f.fainelli@gmail.com>
> Link: http://lore.kernel.org/linux-arm-kernel/2d9284c3-3805-402b-5423-520ced56d047@gmail.com
> Signed-off-by: Mark Rutland <mark.rutland@arm.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Marc Zyngier <maz@kernel.org>
> Cc: Stefan Wahren <stefan.wahren@i2se.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Will Deacon <will@kernel.org>

Tested-by: Florian Fainelli <f.fainelli@gmail.com>

Thanks Mark!
-- 
Florian

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Re: [PATCH] arm64: ftrace: forbid CALL_OPS with CC_OPTIMIZE_FOR_SIZE

[Catalin Marinas]

On Mon, 27 Feb 2023 11:58:19 +0000, Mark Rutland wrote:
> Florian reports that when building with CONFIG_CC_OPTIMIZE_FOR_SIZE=y,
> he sees "Misaligned patch-site" warnings at boot, e.g.
> 
> | Misaligned patch-site bcm2836_arm_irqchip_handle_irq+0x0/0x88
> | WARNING: CPU: 0 PID: 0 at arch/arm64/kernel/ftrace.c:120 ftrace_call_adjust+0x4c/0x70
> 
> This is because GCC will silently ignore `-falign-functions=N` when
> passed `-Os`, resulting in functions not being aligned as we expect.
> This is a known issue, and to account for this we modified the kernel to
> avoid `-Os` generally. Unfortunately we forgot to account for
> CONFIG_CC_OPTIMIZE_FOR_SIZE.
> 
> [...]

Applied to arm64 (for-next/core), thanks!

[1/1] arm64: ftrace: forbid CALL_OPS with CC_OPTIMIZE_FOR_SIZE
      https://git.kernel.org/arm64/c/b3f11af9b2ce

-- 
Catalin


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