Re: [PATCH] powerpc/fadump: fix integer overflow in MIN_RMA size check

[Sayali Patil <sayalip@linux.ibm.com>]

On 20/05/26 14:30, Christophe Leroy (CS GROUP) wrote:
> 
> 
> Le 20/05/2026 à 07:53, Sayali Patil a écrit :
>> The MIN_RMA size checks in fadump_setup_param_area() use
>> (MIN_RMA * 1024 * 1024), which is evaluated in int and can
>> overflow when MIN_RMA is 2048 or larger. This triggers compiler
>> warnings such as:
>>
>> warning: integer overflow in expression of type 'int'
>> results in '0' [-Woverflow]
>>
>> Promote MIN_RMA to u64 before the multiplication so the expression
>> is evaluated in 64-bit and matches the surrounding physical address
>> and memory size calculations.
>>
>> This fixes both the comparison against ppc64_rma_size and the
>> assignment to range_start.
> 
> How do you create that problem ? MIN_RMA has a fixed value of 768 which 
> is defined in asm/prom.h
Agreed, MIN_RMA is currently fixed at 768MB.
While going through the code, I thought this could become a
problem if MIN_RMA is increased beyond values such as SZ_2G.

> 
>>
>> Fixes: b7bb46062457 ("powerpc/fadump: fix additional param memory 
>> reservation for HASH MMU")
>> Signed-off-by: Sayali Patil <sayalip@linux.ibm.com>
>> ---
>>   arch/powerpc/kernel/fadump.c | 4 ++--
>>   1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/powerpc/kernel/fadump.c b/arch/powerpc/kernel/fadump.c
>> index 501d43bf18f3..dea7f7105e42 100644
>> --- a/arch/powerpc/kernel/fadump.c
>> +++ b/arch/powerpc/kernel/fadump.c
>> @@ -1759,10 +1759,10 @@ void __init fadump_setup_param_area(void)
>>            * 2. The range should be between MIN_RMA and RMA size 
>> (ppc64_rma_size)
>>            * 3. It must not overlap with the fadump reserved area.
>>            */
>> -        if (ppc64_rma_size < MIN_RMA*1024*1024)
>> +        if (ppc64_rma_size < (u64)MIN_RMA * 1024 * 1024)
> 
> As you are modifying that line, please use SZ_1M instead of keeping 
> opencoded 1024 * 1024.
> 
>>               return;
>> -        range_start = MIN_RMA * 1024 * 1024;
>> +        range_start = (u64)MIN_RMA * 1024 * 1024;
> 
> Same.
> 
>>           range_end = min(ppc64_rma_size, fw_dump.boot_mem_top);
>>       }
> 
> Maybe the best would be to define MIN_RMA as a bytes value in asm/prom.h 
> and to divide the value by SZ_1M in kernel/prom_init.c when initialising 
> ibm_architecture_vec_template.
> 
> That way you could just define it as SZ_2G instead of the problematic 
> 2048 you mention.
> 
> 
Thanks for the review Christophe!
I will incorporate all the suggested improvements in v2.

Regards,
Sayali