[PATCH] powerpc32/modules: Reserve a PLT entry for ftrace_regs_caller

[PATCH] powerpc32/modules: Reserve a PLT entry for ftrace_regs_caller

[Bradley Morgan]

Commit 7dfbfb87c243 ("powerpc/ftrace: Activate
HAVE_DYNAMIC_FTRACE_WITH_REGS on PPC32") made count_relocs()
undercount the PLT entries the loader
consumes: module_finalize_ftrace() now creates two trampolines
(mod->arch.tramp for ftrace_caller() and mod->arch.tramp_regs for
ftrace_regs_caller()), while count_relocs() reserves only one entry
for ftrace.

So the core .plt ends up one ppc_plt_entry (16 bytes) short. The
walk in do_plt_call() scans for a free slot with
"while (entry->jump[0])" and has no bound check, so once the last
reserved slot is used up, the walk steps past the end of the section
and patch_instruction() writes the 4-instruction trampoline into
whatever module memory follows the .plt.

Count the second entry, mirroring what module_64.c does for the same
pair of stubs.

Signed-off-by: Bradley Morgan <brads@mainlining.org>
---
 arch/powerpc/kernel/module_32.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/powerpc/kernel/module_32.c b/arch/powerpc/kernel/module_32.c
index f930e3395a7f..61152deffc55 100644
--- a/arch/powerpc/kernel/module_32.c
+++ b/arch/powerpc/kernel/module_32.c
@@ -41,6 +41,9 @@ static unsigned int count_relocs(const Elf32_Rela *rela, unsigned int num)
 
 #ifdef CONFIG_DYNAMIC_FTRACE
 	_count_relocs++;	/* add one for ftrace_caller */
+#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
+	_count_relocs++;	/* add one for ftrace_regs_caller */
+#endif
 #endif
 	return _count_relocs;
 }
-- 
2.53.0

Re: [PATCH] powerpc32/modules: Reserve a PLT entry for ftrace_regs_caller

[Christophe Leroy (CS GROUP)]

Le 23/09/2026 à 19:56, Bradley Morgan a écrit :
> Commit 7dfbfb87c243 ("powerpc/ftrace: Activate
> HAVE_DYNAMIC_FTRACE_WITH_REGS on PPC32") made count_relocs()
> undercount the PLT entries the loader
> consumes: module_finalize_ftrace() now creates two trampolines
> (mod->arch.tramp for ftrace_caller() and mod->arch.tramp_regs for
> ftrace_regs_caller()), while count_relocs() reserves only one entry
> for ftrace.
> 
> So the core .plt ends up one ppc_plt_entry (16 bytes) short. The
> walk in do_plt_call() scans for a free slot with
> "while (entry->jump[0])" and has no bound check, so once the last
> reserved slot is used up, the walk steps past the end of the section
> and patch_instruction() writes the 4-instruction trampoline into
> whatever module memory follows the .plt.
> 
> Count the second entry, mirroring what module_64.c does for the same
> pair of stubs.
> 
> Signed-off-by: Bradley Morgan <brads@mainlining.org>

I would have had a preference for something similar to commit 
c12cfe9dee07 ("powerpc/module_64: Convert #ifdef to IS_ENABLED()")

Nevertheless,

Reviewed-by: Christophe Leroy (CS GROUP) <chleroy@kernel.org>



> ---
>   arch/powerpc/kernel/module_32.c | 3 +++
>   1 file changed, 3 insertions(+)
> 
> diff --git a/arch/powerpc/kernel/module_32.c b/arch/powerpc/kernel/module_32.c
> index f930e3395a7f..61152deffc55 100644
> --- a/arch/powerpc/kernel/module_32.c
> +++ b/arch/powerpc/kernel/module_32.c
> @@ -41,6 +41,9 @@ static unsigned int count_relocs(const Elf32_Rela *rela, unsigned int num)
>   
>   #ifdef CONFIG_DYNAMIC_FTRACE
>   	_count_relocs++;	/* add one for ftrace_caller */
> +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
> +	_count_relocs++;	/* add one for ftrace_regs_caller */
> +#endif
>   #endif
>   	return _count_relocs;
>   }

Re: [PATCH] powerpc32/modules: Reserve a PLT entry for ftrace_regs_caller

[Bradley Morgan]

On 10 November 2026 08:57:28 GMT, "Christophe Leroy (CS GROUP)"
<chleroy@kernel.org> wrote:
>
>
>Le 23/09/2026 à 19:56, Bradley Morgan a écrit :
>> Commit 7dfbfb87c243 ("powerpc/ftrace: Activate
>> HAVE_DYNAMIC_FTRACE_WITH_REGS on PPC32") made count_relocs()
>> undercount the PLT entries the loader
>> consumes: module_finalize_ftrace() now creates two trampolines
>> (mod->arch.tramp for ftrace_caller() and mod->arch.tramp_regs for
>> ftrace_regs_caller()), while count_relocs() reserves only one entry
>> for ftrace.
>> 
>> So the core .plt ends up one ppc_plt_entry (16 bytes) short. The
>> walk in do_plt_call() scans for a free slot with
>> "while (entry->jump[0])" and has no bound check, so once the last
>> reserved slot is used up, the walk steps past the end of the section
>> and patch_instruction() writes the 4-instruction trampoline into
>> whatever module memory follows the .plt.
>> 
>> Count the second entry, mirroring what module_64.c does for the same
>> pair of stubs.
>> 
>> Signed-off-by: Bradley Morgan <brads@mainlining.org>
>
>I would have had a preference for something similar to commit c12cfe9dee07
>("powerpc/module_64: Convert #ifdef to IS_ENABLED()")
>
>Nevertheless,
>
>Reviewed-by: Christophe Leroy (CS GROUP) <chleroy@kernel.org>

Thx, now the good question will be, who merges?

>
>
>
>> ---
>>   arch/powerpc/kernel/module_32.c | 3 +++
>>   1 file changed, 3 insertions(+)
>> 
>> diff --git a/arch/powerpc/kernel/module_32.c
>b/arch/powerpc/kernel/module_32.c
>> index f930e3395a7f..61152deffc55 100644
>> --- a/arch/powerpc/kernel/module_32.c
>> +++ b/arch/powerpc/kernel/module_32.c
>> @@ -41,6 +41,9 @@ static unsigned int count_relocs(const Elf32_Rela
>*rela, unsigned int num)
>>     #ifdef CONFIG_DYNAMIC_FTRACE
>>   	_count_relocs++;	/* add one for ftrace_caller */
>> +#ifdef CONFIG_DYNAMIC_FTRACE_WITH_REGS
>> +	_count_relocs++;	/* add one for ftrace_regs_caller */
>> +#endif
>>   #endif
>>   	return _count_relocs;
>>   }
>

--- Thanks!