linux-kernel - Re: [RFC PATCH 01/11] efi: Fix gcc error around _

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Message-ID: <5e2ce2ae-5458-8579-576a-76721f7d3b08@gmx.de>
Date:   Fri, 26 Jun 2020 04:42:46 +0200
From:   Heinrich Schuchardt <xypron.glpk@....de>
To:     Atish Patra <atish.patra@....com>, linux-kernel@...r.kernel.org
Cc:     Ard Biesheuvel <ardb@...nel.org>, linux-efi@...r.kernel.org,
        linux-riscv@...ts.infradead.org,
        Masahiro Yamada <masahiroy@...nel.org>,
        Palmer Dabbelt <palmer@...belt.com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC PATCH 01/11] efi: Fix gcc error around __umoddi3 for 32 bit
 builds

On 6/26/20 1:45 AM, Atish Patra wrote:
> 32bit gcc doesn't support modulo operation on 64 bit data. It results in
> a __umoddi3 error while building EFI for 32 bit.
>
> Use bitwise operations instead of modulo operations to fix the issue.
>
> Signed-off-by: Atish Patra <atish.patra@....com>
> ---
>  drivers/firmware/efi/libstub/alignedmem.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/firmware/efi/libstub/alignedmem.c b/drivers/firmware/efi/libstub/alignedmem.c
> index cc89c4d6196f..1de9878ddd3a 100644
> --- a/drivers/firmware/efi/libstub/alignedmem.c
> +++ b/drivers/firmware/efi/libstub/alignedmem.c
> @@ -44,7 +44,7 @@ efi_status_t efi_allocate_pages_aligned(unsigned long size, unsigned long *addr,
>  	*addr = ALIGN((unsigned long)alloc_addr, align);
>
>  	if (slack > 0) {
> -		int l = (alloc_addr % align) / EFI_PAGE_SIZE;
> +		int l = (alloc_addr & (align - 1)) / EFI_PAGE_SIZE;

Here you rely on the compiler to silently convert the division by
EFI_PAGE_SIZE into a right shift. Wouldn't it be cleaner to use
EFI_PAGE_SHIFT to explicitly avoid a dependency on __udivdi3()?

slack = (align >> EFI_PAGE_SHIFT) - 1;
...
int l = (alloc_addr & (align - 1)) >> EFI_PAGE_SHIFT;

Best regards

Heinrich

>
>  		if (l) {
>  			efi_bs_call(free_pages, alloc_addr, slack - l + 1);
>