On Thu, Sep 16, 2021 at 11:37:53AM +0200, Ahmad Fatoum wrote: > Many arm32 board entry points use arm_setup_stack() to set up > the stack from C code. This necessitates using __naked, which > probably has been our most frequent cause of misscompiled C code. > > GCC is quite clear that: > > Only basic asm statements can safely be included in naked functions > While using extended asm or a mixture of basic asm and C code may > appear to work, they cannot be depended upon to work reliably and > are not supported. > > But some boards use it anyway, because it's nice to avoid writing > assembly. Reading generated assembly to spot compiler miscompilation > isn't that nice though, so add some documentation, comments > and compiler diagnostics to hopefully reduce future porting effort. > > Signed-off-by: Ahmad Fatoum <a.fatoum@xxxxxxxxxxxxxx> > --- > v1 -> v2: > - fix commit message title > > Cc: Jules Maselbas <jmaselbas@xxxxxxxxx> > --- > Documentation/devel/porting.rst | 21 +++++++++++++++++++++ > arch/arm/include/asm/common.h | 22 ++++++++++++++++++++++ > 2 files changed, 43 insertions(+) > > diff --git a/Documentation/devel/porting.rst b/Documentation/devel/porting.rst > index 97b787327c9a..699badbec672 100644 > --- a/Documentation/devel/porting.rst > +++ b/Documentation/devel/porting.rst > @@ -169,6 +169,27 @@ Looking at other boards you might see some different patterns: > needs to be done at start. If a board similar to yours does this, you probably > want to do likewise. > > + - ``__naked``: All functions called before stack is correctly initialized must be > + marked with this attribute. Otherwise, function prologue and epilogue may access > + the uninitialized stack. If the compiler for the target architecture doesn't > + support the attribute, stack must be set up in non-inline assembly: > + either a barebox assembly entry point or in earlier firmware. s/either/Either/ > + The compiler may still spill excess local C variables used in a naked function > + to the stack before it was initialized. > + A naked function should thus preferably only contain inline assembly, set up a > + stack and jump directly after to a ``noinline`` non naked function where the > + stack is then normally usable. > + > + - ``noinline``: Compiler code inlining is oblivious to stack modification. > + If you want to ensure a new function has its own stack frame (e.g. after setting > + up the stack in a ``__naked`` function), you must jump to a ``__noreturn noinline`` > + function. > + > + - ``arm_setup_stack``: For 32-bit ARM, ``arm_setup_stack`` initialized the stack s/initialized/initializes/ > + top when called from a naked C function, which allows to write the entry point > + directly in C. The stack pointer will be decremented before pushing values. > + Avoid interleaving with C-code. See ``__naked`` above for more details. > + > - ``__dtb_z_my_board_start[];``: Because the PBL normally doesn't parse anything out > of the device tree blob, boards can benefit from keeping the device tree blob > compressed and only unpack it in barebox proper. Such LZO-compressed device trees > diff --git a/arch/arm/include/asm/common.h b/arch/arm/include/asm/common.h > index d03ee6273fe5..88e2991c9324 100644 > --- a/arch/arm/include/asm/common.h > +++ b/arch/arm/include/asm/common.h > @@ -1,6 +1,8 @@ > #ifndef __ASM_ARM_COMMON_H > #define __ASM_ARM_COMMON_H > > +#include <linux/compiler.h> > + > static inline unsigned long get_pc(void) > { > unsigned long pc; > @@ -46,8 +48,28 @@ static inline unsigned long get_sp(void) > return sp; > } > > +extern void __compiletime_error( > + "arm_setup_stack() called outside of naked function. On ARM64, " > + "stack should be setup in non-inline assembly before branching to C entry point." > +) __unsafe_stack_setup(void); > + > +/* > + * Sets up new stack growing down from top within a naked C function. > + * The first stack word will be top - sizeof(word). > + * > + * Avoid interleaving with C code as much as possible and and jump s/and and/and/ > + * ASAP to a noinline function. > + */ > static inline void arm_setup_stack(unsigned long top) > { > + if (IS_ENABLED(CONFIG_CPU_V8)) { For configs that have CONFIG_CPU_V8 and CONFIG_CPU_V7 it might be legal to call arm_setup_stack when running on a v7 SoC. Not sure how relevant this is though, maybe just add a comment? > + /* > + * There are no naked functions on ARM64 and thus it's never > + * safe to call arm_setup_stack(). > + */ > + __unsafe_stack_setup(); > + } > + Best regards Uwe -- Pengutronix e.K. | Uwe Kleine-König | Industrial Linux Solutions | https://www.pengutronix.de/ |
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