kbuild: rust: keep Rust objects out of Clang LTO with inline helpersgrafted

Under `CONFIG_LTO_CLANG` + `CONFIG_RUST_INLINE_HELPERS`, one may hit
`objtool` errors such as:

    vmlinux.o: error: objtool: _R..._3Gsp4boot+0xd6a:
    can't find jump dest instruction at .text._R..._3Gsp4boot+0x1dfd

The reason is that in such builds, the Clang invocation that compiles
the combined Rust plus helpers bitcode emits LLVM bitcode (again) --
the final code generation happens in the linker's LTO step, which the
`-mllvm` trap options passed to Clang do not reach.

This, in turn, means that unreachable traps are missing, and the
impossible paths do not merely fallthrough to the next symbol, but past
the end of their own section, since LTO builds place each function in
its own section.

Thus filter `CC_FLAGS_LTO` out of the Clang invocation, so that it always
emits machine code directly, with the traps in place.

Assisted-by: LLM
Cc: Gary Guo <gary@garyguo.net>
Cc: Boqun Feng <boqun@kernel.org>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Matthew Maurer <mmaurer@google.com>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org
Fixes: 3a2486cc1da5 ("kbuild: rust: provide an option to inline C helpers into Rust")
Acked-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260816133233.197500-2-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 186 insertions, 0 deletions

diff --git a/Documentation/ABI/testing/sysfs-class-backlight b/Documentation/ABI/testing/sysfs-class-backlight
new file mode 100644
index 000000000..c453646b0
--- /dev/null
+++ b/Documentation/ABI/testing/sysfs-class-backlight
@@ -0,0 +1,186 @@
+What:		/sys/class/backlight/<backlight>/scale
+Date:		July 2019
+KernelVersion:	5.4
+Contact:	Daniel Thompson <daniel.thompson@linaro.org>
+Description:
+		Description of the scale of the brightness curve.
+
+		The human eye senses brightness approximately logarithmically,
+		hence linear changes in brightness are perceived as being
+		non-linear. To achieve a linear perception of brightness changes
+		controls like sliders need to apply a logarithmic mapping for
+		backlights with a linear brightness curve.
+
+		Possible values of the attribute are:
+
+		unknown
+		  The scale of the brightness curve is unknown.
+
+		linear
+		  The brightness changes linearly with each step. Brightness
+		  controls should apply a logarithmic mapping for a linear
+		  perception.
+
+		non-linear
+		  The brightness changes non-linearly with each step. Brightness
+		  controls should use a linear mapping for a linear perception.
+
+What:		/sys/class/backlight/<backlight>/ambient_light_level
+Date:		Apr, 2010
+KernelVersion:	v2.6.35
+Contact:	Michael Hennerich <michael.hennerich@analog.com>
+Description:
+		(RO) Get conversion value of the light sensor.
+
+		The value is automatically updated every 80 ms when the
+		light sensor is enabled.
+
+		The value range is device-driver specific:
+
+		For ADP8870:
+
+		  It returns integer between 0 (dark) and 8000 (max ambient
+		  brightness).
+
+		For ADP8860:
+
+		  It returns a 13-bits integer.
+
+What:		/sys/class/backlight/<backlight>/ambient_light_zone
+Date:		Apr, 2010
+KernelVersion:	v2.6.35
+Contact:	Michael Hennerich <michael.hennerich@analog.com>,
+		device-drivers-devel@blackfin.uclinux.org
+
+Description:
+		(RW) Read or write the specific brightness level at which the
+		backlight operates.
+
+		The value meaning is device-driver specific:
+
+		For ADP8860:
+
+		  ==	==========================
+		   0	Off: Backlight set to 0 mA
+		   1	Level 1: daylight
+		   2	Level 2: bright
+		   3	Level 3: dark
+		  ==	==========================
+
+		For ADP8870:
+
+		  ==	==========================
+		   0	Off: Backlight set to 0 mA
+		   1	Level 1: daylight
+		   2	Level 2: bright
+		   3	Level 3: office
+		   4	Level 4: indoor
+		   5	Level 5: dark
+		  ==	==========================
+
+		Writing 0 returns to normal/automatic ambient light level
+		operation.
+
+		It can be enabled by writing the value stored in
+		/sys/class/backlight/<backlight>/max_brightness to
+		/sys/class/backlight/<backlight>/brightness.
+
+What:		/sys/class/backlight/<backlight>/<ambient light zone>_max
+Date:		Sep, 2009
+KernelVersion:	v2.6.32
+Contact:	device-drivers-devel@blackfin.uclinux.org
+Description:
+		Control the maximum brightness for <ambient light zone>
+		on this <backlight>. Values are between 0 and 127. This file
+		will also show the brightness level stored for this
+		<ambient light zone>.
+
+		The <ambient light zone> is device-driver specific:
+
+		For ADP5520 and ADP5501, <ambient light zone> can be:
+
+		===========  ================================================
+		Ambient      sysfs entry
+		light zone
+		===========  ================================================
+		daylight     /sys/class/backlight/<backlight>/daylight_max
+		office       /sys/class/backlight/<backlight>/office_max
+		dark         /sys/class/backlight/<backlight>/dark_max
+		===========  ================================================
+
+		For ADP8860, <ambient light zone> can be:
+
+		===========  ================================================
+		Ambient      sysfs entry
+		light zone
+		===========  ================================================
+		l1_daylight  /sys/class/backlight/<backlight>/l1_daylight_max
+		l2_office    /sys/class/backlight/<backlight>/l2_office_max
+		l3_dark      /sys/class/backlight/<backlight>/l3_dark_max
+		===========  ================================================
+
+		For ADP8870, <ambient light zone> can be:
+
+		===========  ================================================
+		Ambient      sysfs entry
+		light zone
+		===========  ================================================
+		l1_daylight  /sys/class/backlight/<backlight>/l1_daylight_max
+		l2_bright    /sys/class/backlight/<backlight>/l2_bright_max
+		l3_office    /sys/class/backlight/<backlight>/l3_office_max
+		l4_indoor    /sys/class/backlight/<backlight>/l4_indoor_max
+		l5_dark      /sys/class/backlight/<backlight>/l5_dark_max
+		===========  ================================================
+
+		See also: /sys/class/backlight/<backlight>/ambient_light_zone.
+
+What:		/sys/class/backlight/<backlight>/<ambient light zone>_dim
+Date:		Sep, 2009
+KernelVersion:	v2.6.32
+Contact:	device-drivers-devel@blackfin.uclinux.org
+Description:
+		Control the dim brightness for <ambient light zone>
+		on this <backlight>. Values are between 0 and 127, typically
+		set to 0. Full off when the backlight is disabled.
+		This file will also show the dim brightness level stored for
+		this <ambient light zone>.
+
+		The <ambient light zone> is device-driver specific:
+
+		For ADP5520 and ADP5501, <ambient light zone> can be:
+
+		===========  ================================================
+		Ambient      sysfs entry
+		light zone
+		===========  ================================================
+		daylight     /sys/class/backlight/<backlight>/daylight_dim
+		office       /sys/class/backlight/<backlight>/office_dim
+		dark         /sys/class/backlight/<backlight>/dark_dim
+		===========  ================================================
+
+		For ADP8860, <ambient light zone> can be:
+
+		===========  ================================================
+		Ambient      sysfs entry
+		light zone
+		===========  ================================================
+		l1_daylight  /sys/class/backlight/<backlight>/l1_daylight_dim
+		l2_office    /sys/class/backlight/<backlight>/l2_office_dim
+		l3_dark      /sys/class/backlight/<backlight>/l3_dark_dim
+		===========  ================================================
+
+		For ADP8870, <ambient light zone> can be:
+
+		===========  ================================================
+		Ambient      sysfs entry
+		light zone
+		===========  ================================================
+		l1_daylight  /sys/class/backlight/<backlight>/l1_daylight_dim
+		l2_bright    /sys/class/backlight/<backlight>/l2_bright_dim
+		l3_office    /sys/class/backlight/<backlight>/l3_office_dim
+		l4_indoor    /sys/class/backlight/<backlight>/l4_indoor_dim
+		l5_dark      /sys/class/backlight/<backlight>/l5_dark_dim
+		===========  ================================================
+
+		See also: /sys/class/backlight/<backlight>/ambient_light_zone.
+