linux-kernel - Re: [PATCH v2] Fix display of Maximum Memory

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Message-ID: <8736cg9cay.fsf@mpe.ellerman.id.au>
Date:   Thu, 16 Jan 2020 15:53:57 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Michael Bringmann <mwb@...ux.ibm.com>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        Gustavo Walbon <gwalbon@...ux.ibm.com>,
        Paul Mackerras <paulus@...ba.org>
Subject: Re: [PATCH v2] Fix display of Maximum Memory

Michael Bringmann <mwb@...ux.ibm.com> writes:
> Correct overflow problem in calculation+display of Maximum Memory
> value to syscfg where 32bits is insufficient.
>
> Signed-off-by: Michael Bringmann <mwb@...ux.ibm.com>
> ---
>  arch/powerpc/platforms/pseries/lparcfg.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/arch/powerpc/platforms/pseries/lparcfg.c b/arch/powerpc/platforms/pseries/lparcfg.c
> index e33e8bc..f00411c 100644
> --- a/arch/powerpc/platforms/pseries/lparcfg.c
> +++ b/arch/powerpc/platforms/pseries/lparcfg.c
> @@ -433,12 +433,12 @@ static void parse_em_data(struct seq_file *m)
>  
>  static void maxmem_data(struct seq_file *m)
>  {
> -	unsigned long maxmem = 0;
> +	u64 maxmem = 0;

This is 64-bit only code, so u64 == unsigned long.

> -	maxmem += drmem_info->n_lmbs * drmem_info->lmb_size;
> -	maxmem += hugetlb_total_pages() * PAGE_SIZE;
> +	maxmem += (u64)drmem_info->n_lmbs * drmem_info->lmb_size;

The only problem AFAICS is n_lmbs is int and lmb_size is u32, so this
multiplication will overflow.

> +	maxmem += (u64)hugetlb_total_pages() * PAGE_SIZE;

hugetlb_total_pages() already returns unsigned long.

> -	seq_printf(m, "MaxMem=%ld\n", maxmem);
> +	seq_printf(m, "MaxMem=%llu\n", maxmem);
>  }

This should be sufficient?

diff --git a/arch/powerpc/platforms/pseries/lparcfg.c b/arch/powerpc/platforms/pseries/lparcfg.c
index e33e8bc4b69b..38c306551f76 100644
--- a/arch/powerpc/platforms/pseries/lparcfg.c
+++ b/arch/powerpc/platforms/pseries/lparcfg.c
@@ -435,10 +435,10 @@ static void maxmem_data(struct seq_file *m)
 {
        unsigned long maxmem = 0;
 
-       maxmem += drmem_info->n_lmbs * drmem_info->lmb_size;
+       maxmem += (unsigned long)drmem_info->n_lmbs * drmem_info->lmb_size;
        maxmem += hugetlb_total_pages() * PAGE_SIZE;
 
-       seq_printf(m, "MaxMem=%ld\n", maxmem);
+       seq_printf(m, "MaxMem=%lu\n", maxmem);
 }
 
 static int pseries_lparcfg_data(struct seq_file *m, void *v)


cheers