Re: [PATCH] riscv: Prevent early kernel panic in instrumented apply_early_boot_alternatives

[duchangbin <changbin.du@huawei.com>]

On Tue, Jun 24, 2025 at 02:54:32PM +0200, Alexandre Ghiti wrote:
> Hi Changbin,
> 
> On 6/24/25 13:30, Changbin Du wrote:
> > Under FTRACE=y, DYNAMIC_FTRACE=n, and RISCV_ALTERNATIVE_EARLY=n, the kernel
> 
> 
> Your above config works fine for me, I guess you meant FUNCTION_TRACER &&
> !DYNAMIC_FTRACE (which fails).
>
Yes, it's FUNCTION_TRACER.

> We were just talking with Andy about this configuration (FUNCTION_TRACER &&
> !DYNAMIC_FTRACE): do we really want to support static ftrace? Andy should
> send a patch soon to remove this possibility as IMO we don't want to support
> it. Let's wait for this patch and the discussion that will follow before
> merging your fix. I'll keep it in my list for 6.16 just in case someone
> comes up with a good argument to keep it.
> 
No problem. I'm unable to enable DYNAMIC_FTRACE because GCC_SUPPORTS_DYNAMIC_FTRACE=n
and CC_HAS_MIN_FUNCTION_ALIGNMENT=n are set. It seems that my GCC version (13.3.0)
does not support the option -fmin-function-alignment=8.

By the way, this change also eliminates an empty function call.

> Thanks,
> 
> Alex
> 
> 
> > panics upon returning from _mcount() in the early boot path. This occurs
> > during _start_kernel() -> setup_vm() -> apply_early_boot_alternatives().
> > 
> > The CC_FLAGS_FTRACE is only removed from alternative.c when
> > CONFIG_RISCV_ALTERNATIVE_EARLY=y. Therefore, no function calls should be
> > made to alternative.c during early boot in this configuration.
> > 
> > Fixes: a35707c3d850 ("riscv: add memory-type errata for T-Head")
> > Signed-off-by: Changbin Du <changbin.du@huawei.com>
> > ---
> >   arch/riscv/include/asm/alternative.h | 6 ++++++
> >   arch/riscv/kernel/alternative.c      | 4 ++--
> >   2 files changed, 8 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/riscv/include/asm/alternative.h b/arch/riscv/include/asm/alternative.h
> > index 3c2b59b25017..c670b0cc55f4 100644
> > --- a/arch/riscv/include/asm/alternative.h
> > +++ b/arch/riscv/include/asm/alternative.h
> > @@ -31,7 +31,13 @@
> >   #define ALT_ALT_PTR(a)			__ALT_PTR(a, alt_offset)
> >   void __init apply_boot_alternatives(void);
> > +
> > +# ifdef CONFIG_RISCV_ALTERNATIVE_EARLY
> >   void __init apply_early_boot_alternatives(void);
> > +# else
> > +static inline void apply_early_boot_alternatives(void) { }
> > +# endif
> > +
> >   void apply_module_alternatives(void *start, size_t length);
> >   void riscv_alternative_fix_offsets(void *alt_ptr, unsigned int len,
> > diff --git a/arch/riscv/kernel/alternative.c b/arch/riscv/kernel/alternative.c
> > index 7eb3cb1215c6..5406c3301627 100644
> > --- a/arch/riscv/kernel/alternative.c
> > +++ b/arch/riscv/kernel/alternative.c
> > @@ -205,6 +205,7 @@ void __init apply_boot_alternatives(void)
> >   	apply_vdso_alternatives();
> >   }
> > +#ifdef CONFIG_RISCV_ALTERNATIVE_EARLY
> >   /*
> >    * apply_early_boot_alternatives() is called from setup_vm() with MMU-off.
> >    *
> > @@ -219,12 +220,11 @@ void __init apply_boot_alternatives(void)
> >    */
> >   void __init apply_early_boot_alternatives(void)
> >   {
> > -#ifdef CONFIG_RISCV_ALTERNATIVE_EARLY
> >   	_apply_alternatives((struct alt_entry *)__alt_start,
> >   			    (struct alt_entry *)__alt_end,
> >   			    RISCV_ALTERNATIVES_EARLY_BOOT);
> > -#endif
> >   }
> > +#endif
> >   #ifdef CONFIG_MODULES
> >   void apply_module_alternatives(void *start, size_t length)

-- 
Cheers,
Changbin Du