Re: WTF: patch "[PATCH] ARC: Support syscall ABI v4" was seriously submitted to be applied to the 4.7-stable tree?

[Vineet Gupta <Vineet.Gupta1@synopsys.com>]

On 09/05/2016 06:03 AM, gregkh@linuxfoundation.org wrote:
> The patch below was submitted to be applied to the 4.7-stable tree.
>
> I fail to see how this patch meets the stable kernel rules as found at
> Documentation/stable_kernel_rules.txt.
>
> I could be totally wrong, and if so, please respond to 
> <stable@vger.kernel.org> and let me know why this patch should be
> applied.  Otherwise, it is now dropped from my patch queues, never to be
> seen again.

Hi Greg,

We are about to switch to new version of GNU tools (gcc 6.x based) which
unfortunately has a non compatible ABI change - as described in the patch.

Some of our customers are going to stick with older kernels and thus this helps
them upgrade to newer tools with their existing baseline kernels.

Thx,
-Vineet

>
> thanks,
>
> greg k-h
>
> ------------------ original commit in Linus's tree ------------------
>
> From 840c054fd0efb048df6fceb0c46385ec5b66dfe6 Mon Sep 17 00:00:00 2001
> From: Vineet Gupta <vgupta@synopsys.com>
> Date: Wed, 10 Aug 2016 14:10:57 -0700
> Subject: [PATCH] ARC: Support syscall ABI v4
>
> The syscall ABI includes the gcc functional calling ABI since a syscall
> implies userland caller and kernel callee.
>
> The current gcc ABI (v3) for ARCv2 ISA required 64-bit data be passed in
> even-odd register pairs, (potentially punching reg holes when passing such
> values as args). This was partly driven by the fact that the double-word
> LDD/STD instructions in ARCv2 expect the register alignment and thus gcc
> forcing this avoids extra MOV at the cost of a few unused register (which we
> have plenty anyways).
>
> This however was rejected as part of upstreaming gcc port to HS. So the new
> ABI v4 doesn't enforce the even-odd reg restriction.
>
> Do note that for ARCompact ISA builds v3 and v4 are practically the same in
> terms of gcc code generation.
>
> In terms of change management, we infer the new ABI if gcc 6.x onwards
> is used for building the kernel.
>
> This also needs a stable backport to enable older kernels to work with
> new tools/user-space
>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
>
> diff --git a/arch/arc/include/uapi/asm/elf.h b/arch/arc/include/uapi/asm/elf.h
> index 0f99ac8fcbb2..0037a587320d 100644
> --- a/arch/arc/include/uapi/asm/elf.h
> +++ b/arch/arc/include/uapi/asm/elf.h
> @@ -13,8 +13,15 @@
>  
>  /* Machine specific ELF Hdr flags */
>  #define EF_ARC_OSABI_MSK	0x00000f00
> -#define EF_ARC_OSABI_ORIG	0x00000000   /* MUST be zero for back-compat */
> -#define EF_ARC_OSABI_CURRENT	0x00000300   /* v3 (no legacy syscalls) */
> +
> +#define EF_ARC_OSABI_V3		0x00000300   /* v3 (no legacy syscalls) */
> +#define EF_ARC_OSABI_V4		0x00000400   /* v4 (64bit data any reg align) */
> +
> +#if __GNUC__ < 6
> +#define EF_ARC_OSABI_CURRENT	EF_ARC_OSABI_V3
> +#else
> +#define EF_ARC_OSABI_CURRENT	EF_ARC_OSABI_V4
> +#endif
>  
>  typedef unsigned long elf_greg_t;
>  typedef unsigned long elf_fpregset_t;
> diff --git a/arch/arc/kernel/process.c b/arch/arc/kernel/process.c
> index b5db9e7fd649..be1972bd2729 100644
> --- a/arch/arc/kernel/process.c
> +++ b/arch/arc/kernel/process.c
> @@ -199,7 +199,7 @@ int elf_check_arch(const struct elf32_hdr *x)
>  	}
>  
>  	eflags = x->e_flags;
> -	if ((eflags & EF_ARC_OSABI_MSK) < EF_ARC_OSABI_CURRENT) {
> +	if ((eflags & EF_ARC_OSABI_MSK) != EF_ARC_OSABI_CURRENT) {
>  		pr_err("ABI mismatch - you need newer toolchain\n");
>  		force_sigsegv(SIGSEGV, current);
>  		return 0;
> diff --git a/arch/arc/kernel/setup.c b/arch/arc/kernel/setup.c
> index a946400a86d0..f52a0d0dc462 100644
> --- a/arch/arc/kernel/setup.c
> +++ b/arch/arc/kernel/setup.c
> @@ -291,8 +291,10 @@ static char *arc_extn_mumbojumbo(int cpu_id, char *buf, int len)
>  			       cpu->dccm.base_addr, TO_KB(cpu->dccm.sz),
>  			       cpu->iccm.base_addr, TO_KB(cpu->iccm.sz));
>  
> -	n += scnprintf(buf + n, len - n,
> -		       "OS ABI [v3]\t: no-legacy-syscalls\n");
> +	n += scnprintf(buf + n, len - n, "OS ABI [v%d]\t: %s\n",
> +			EF_ARC_OSABI_CURRENT >> 8,
> +			EF_ARC_OSABI_CURRENT == EF_ARC_OSABI_V3 ?
> +			"no-legacy-syscalls" : "64-bit data any register aligned");
>  
>  	return buf;
>  }
>
>