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Message-ID: <20160104120751.GG22941@pd.tnic>
Date: Mon, 4 Jan 2016 13:07:51 +0100
From: Borislav Petkov <bp@...en8.de>
To: Andy Lutomirski <luto@...capital.net>
Cc: Ingo Molnar <mingo@...nel.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Andy Lutomirski <luto@...nel.org>,
Dan Williams <dan.j.williams@...el.com>, elliott@....com,
linux-kernel@...r.kernel.org, linux-mm@...ck.org,
linux-nvdimm@...1.01.org, x86@...nel.org
Subject: Re: [PATCH v6 1/4] x86: Clean up extable entry format (and free up a
bit)
On Wed, Dec 30, 2015 at 09:59:29AM -0800, Andy Lutomirski wrote:
> This adds two bits of fixup class information to a fixup entry,
> generalizing the uaccess_err hack currently in place.
>
> Forward-ported-from-3.9-by: Tony Luck <tony.luck@...el.com>
> Signed-off-by: Andy Lutomirski <luto@...capital.net>
> ---
> arch/x86/include/asm/asm.h | 70 ++++++++++++++++++++++++++++++----------------
> arch/x86/mm/extable.c | 21 ++++++++------
> 2 files changed, 59 insertions(+), 32 deletions(-)
>
> diff --git a/arch/x86/include/asm/asm.h b/arch/x86/include/asm/asm.h
> index 189679aba703..b64121ffb2da 100644
> --- a/arch/x86/include/asm/asm.h
> +++ b/arch/x86/include/asm/asm.h
> @@ -43,19 +43,47 @@
> #define _ASM_DI __ASM_REG(di)
>
> /* Exception table entry */
> -#ifdef __ASSEMBLY__
> -# define _ASM_EXTABLE(from,to) \
> - .pushsection "__ex_table","a" ; \
> - .balign 8 ; \
> - .long (from) - . ; \
> - .long (to) - . ; \
> - .popsection
>
> -# define _ASM_EXTABLE_EX(from,to) \
> - .pushsection "__ex_table","a" ; \
> - .balign 8 ; \
> - .long (from) - . ; \
> - .long (to) - . + 0x7ffffff0 ; \
> +/*
> + * An exception table entry is 64 bits. The first 32 bits are the offset
Two 32-bit ints, to be exact.
Also, there's text in arch/x86/include/asm/uaccess.h where the exception
table entry is defined so you probably should sync with it so that the
nomenclature is the same.
> + * from that entry to the potentially faulting instruction. sortextable
sortextable.c ?
> + * relies on that exact encoding. The second 32 bits encode the fault
> + * handler address.
> + *
> + * We want to stick two extra bits of handler class into the fault handler
> + * address. All of these are generated by relocations, so we can only
> + * rely on addition. We therefore emit:
> + *
> + * (target - here) + (class) + 0x20000000
I still don't understand that bit 29 thing.
Because the offset is negative?
The exception table currently looks like this here:
insn offset: 0xff91a7c4, fixup offset: 0xffffd57a
insn offset: 0xff91bac3, fixup offset: 0xffffd57e
insn offset: 0xff91bac0, fixup offset: 0xffffd57d
insn offset: 0xff91baba, fixup offset: 0xffffd57c
insn offset: 0xff91bfca, fixup offset: 0xffffd57c
insn offset: 0xff91bfff, fixup offset: 0xffffd57e
insn offset: 0xff91c049, fixup offset: 0xffffd580
insn offset: 0xff91c141, fixup offset: 0xffffd57f
insn offset: 0xff91c24e, fixup offset: 0xffffd581
insn offset: 0xff91c262, fixup offset: 0xffffd580
insn offset: 0xff91c261, fixup offset: 0xffffd57f
...
It probably will dawn on me when I look at the rest of the patch...
> + * This has the property that the two high bits are the class and the
> + * rest is easy to decode.
> + */
> +
> +/* There are two bits of extable entry class, added to a signed offset. */
> +#define _EXTABLE_CLASS_DEFAULT 0 /* standard uaccess fixup */
> +#define _EXTABLE_CLASS_EX 0x80000000 /* uaccess + set uaccess_err */
BIT(31) is more readable.
> +
> +/*
> + * The biases are the class constants + 0x20000000, as signed integers.
> + * This can't use ordinary arithmetic -- the assembler isn't that smart.
> + */
> +#define _EXTABLE_BIAS_DEFAULT 0x20000000
> +#define _EXTABLE_BIAS_EX 0x20000000 - 0x80000000
Ditto.
> +
> +#define _ASM_EXTABLE(from,to) \
> + _ASM_EXTABLE_CLASS(from, to, _EXTABLE_BIAS_DEFAULT)
> +
> +#define _ASM_EXTABLE_EX(from,to) \
> + _ASM_EXTABLE_CLASS(from, to, _EXTABLE_BIAS_EX)
> +
> +#ifdef __ASSEMBLY__
> +# define _EXPAND_EXTABLE_BIAS(x) x
> +# define _ASM_EXTABLE_CLASS(from,to,bias) \
> + .pushsection "__ex_table","a" ; \
> + .balign 8 ; \
> + .long (from) - . ; \
> + .long (to) - . + _EXPAND_EXTABLE_BIAS(bias) ; \
Why not simply:
.long (to) - . + (bias) ;
and
" .long (" #to ") - . + "(" #bias ") "\n"
below and get rid of that _EXPAND_EXTABLE_BIAS()?
> .popsection
>
> # define _ASM_NOKPROBE(entry) \
> @@ -89,18 +117,12 @@
> .endm
>
> #else
> -# define _ASM_EXTABLE(from,to) \
> - " .pushsection \"__ex_table\",\"a\"\n" \
> - " .balign 8\n" \
> - " .long (" #from ") - .\n" \
> - " .long (" #to ") - .\n" \
> - " .popsection\n"
> -
> -# define _ASM_EXTABLE_EX(from,to) \
> - " .pushsection \"__ex_table\",\"a\"\n" \
> - " .balign 8\n" \
> - " .long (" #from ") - .\n" \
> - " .long (" #to ") - . + 0x7ffffff0\n" \
> +# define _EXPAND_EXTABLE_BIAS(x) #x
> +# define _ASM_EXTABLE_CLASS(from,to,bias) \
> + " .pushsection \"__ex_table\",\"a\"\n" \
> + " .balign 8\n" \
> + " .long (" #from ") - .\n" \
> + " .long (" #to ") - . + " _EXPAND_EXTABLE_BIAS(bias) "\n" \
> " .popsection\n"
> /* For C file, we already have NOKPROBE_SYMBOL macro */
> #endif
> diff --git a/arch/x86/mm/extable.c b/arch/x86/mm/extable.c
> index 903ec1e9c326..95e2ede71206 100644
> --- a/arch/x86/mm/extable.c
> +++ b/arch/x86/mm/extable.c
> @@ -8,16 +8,24 @@ ex_insn_addr(const struct exception_table_entry *x)
> {
> return (unsigned long)&x->insn + x->insn;
> }
> +static inline unsigned int
> +ex_class(const struct exception_table_entry *x)
> +{
> + return (unsigned int)x->fixup & 0xC0000000;
> +}
> +
> static inline unsigned long
> ex_fixup_addr(const struct exception_table_entry *x)
> {
> - return (unsigned long)&x->fixup + x->fixup;
> + long offset = (long)((u32)x->fixup & 0x3fffffff) - (long)0x20000000;
So basically:
x->fixup & 0x1fffffff
Why the explicit subtraction of bit 29?
IOW, I was expecting something simpler for the whole scheme like:
ex_class:
return x->fixup & 0xC0000000;
ex_fixup_addr:
return x->fixup | 0xC0000000;
Why can't it be done this way?
--
Regards/Gruss,
Boris.
ECO tip #101: Trim your mails when you reply.
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