Re: [PATCH] powerpc/ptrace: Add offsetof check for exit_flags in pt_regs_check()

["Christophe Leroy (CS GROUP)" <chleroy@kernel.org>]

Le 03/07/2026 à 10:55, Amit Machhiwal a écrit :
> On 2026/07/03 01:19 PM, Mukesh Kumar Chaurasiya (IBM) wrote:
>> pt_regs_check() verifies that fields shared between struct pt_regs and
>> struct user_pt_regs sit at the same offset, to catch any accidental
>> layout divergence between the kernel and uapi structures.
>>
>> Add the missing check for exit_flags, following the same pattern as the
>> existing checks for result, dsisr, dar and the other shared fields.
>>
>> Fixes: d7a6797e0bc1 ("powerpc: add exit_flags field in pt_regs")

This commit was almost reverted by commit 263e5159e00a ("powerpc: Fix 
exit_flags field placement in pt_regs for ptrace")

Dropping this patch.

Christophe

> 
> You may want to add below in order to get this patch backported:
> 
>    Cc: stable@vger.kernel.org
> 
>> Signed-off-by: Mukesh Kumar Chaurasiya (IBM) <mkchauras@gmail.com>
>> ---
>>   arch/powerpc/kernel/ptrace/ptrace.c | 2 ++
>>   1 file changed, 2 insertions(+)
>>
>> diff --git a/arch/powerpc/kernel/ptrace/ptrace.c b/arch/powerpc/kernel/ptrace/ptrace.c
>> index 316d4f5ead8e..fb3fec73f66a 100644
>> --- a/arch/powerpc/kernel/ptrace/ptrace.c
>> +++ b/arch/powerpc/kernel/ptrace/ptrace.c
>> @@ -235,6 +235,8 @@ void __init pt_regs_check(void)
>>   		     offsetof(struct user_pt_regs, dsisr));
>>   	BUILD_BUG_ON(offsetof(struct pt_regs, result) !=
>>   		     offsetof(struct user_pt_regs, result));
>> +	BUILD_BUG_ON(offsetof(struct pt_regs, exit_flags) !=
>> +		     offsetof(struct user_pt_regs, exit_flags));
> 
> At first, it appeared to be redundant as we already check
> 
> 	CHECK_REG(PT_EXIT_FLAGS, exit_flags);
> 
> which expands to:
> 
> 	#define CHECK_REG(_pt, _reg) \
> 		BUILD_BUG_ON(_pt != (offsetof(struct user_pt_regs, _reg) / \
> 				     sizeof(unsigned long)));
> 
> But looking closely, it turns out to be non-redundant. CHECK_REG only
> touches struct user_pt_regs — it validates the PT_EXIT_FLAGS constant
> against the uapi layout. It says nothing about struct pt_regs.
> 
> struct pt_regs (asm/ptrace.h) embeds struct user_pt_regs in a union
> alongside an anonymous struct that re-declares all the same fields with
> kernel-internal aliases (dear/dar, esr/dsisr). exit_flags lives in that
> anonymous struct. A field accidentally inserted there between result and
> exit_flags would silently break the union aliasing without CHECK_REG
> catching it. The new BUILD_BUG_ON closes that gap, consistent with how
> result, dar and dsisr are already protected in the same block.
> 
> Looks good to me.
> 
> Reviewed-by: Amit Machhiwal <amachhiw@linux.ibm.com>
> 
> Thanks,
> Amit
> 
>>   
>>   	BUILD_BUG_ON(sizeof(struct user_pt_regs) > sizeof(struct pt_regs));
>>   
>> -- 
>> 2.55.0
>>
>>