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Date: Wed, 3 Feb 2016 22:11:38 +0100
From: Borislav Petkov <bp@...en8.de>
To: Tony Luck <tony.luck@...el.com>
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,
Brian Gerst <brgerst@...il.com>, linux-kernel@...r.kernel.org,
linux-mm@...ck.org, linux-nvdimm@...1.01.org, x86@...nel.org
Subject: Re: [PATCH v9 1/4] x86: Expand exception table to allow new handling
options
On Fri, Jan 08, 2016 at 12:49:38PM -0800, Tony Luck wrote:
> Huge amounts of help from Andy Lutomirski and Borislav Petkov to
> produce this. Andy provided the inspiration to add classes to the
> exception table with a clever bit-squeezing trick, Boris pointed
> out how much cleaner it would all be if we just had a new field.
>
> Linus Torvalds blessed the expansion with:
> I'd rather not be clever in order to save just a tiny amount of space
> in the exception table, which isn't really criticial for anybody.
>
> The third field is another relative function pointer, this one to a
> handler that executes the actions.
>
> We start out with three handlers:
>
> 1: Legacy - just jumps the to fixup IP
> 2: Fault - provide the trap number in %ax to the fixup code
> 3: Cleaned up legacy for the uaccess error hack
>
> Signed-off-by: Tony Luck <tony.luck@...el.com>
> ---
> Documentation/x86/exception-tables.txt | 33 +++++++++++
> arch/x86/include/asm/asm.h | 40 +++++++------
> arch/x86/include/asm/uaccess.h | 16 +++---
> arch/x86/kernel/kprobes/core.c | 2 +-
> arch/x86/kernel/traps.c | 6 +-
> arch/x86/mm/extable.c | 100 ++++++++++++++++++++++++---------
> arch/x86/mm/fault.c | 2 +-
> scripts/sortextable.c | 32 +++++++++++
> 8 files changed, 174 insertions(+), 57 deletions(-)
>
> diff --git a/Documentation/x86/exception-tables.txt b/Documentation/x86/exception-tables.txt
> index 32901aa36f0a..d4ca5f8b22ff 100644
> --- a/Documentation/x86/exception-tables.txt
> +++ b/Documentation/x86/exception-tables.txt
> @@ -290,3 +290,36 @@ Due to the way that the exception table is built and needs to be ordered,
> only use exceptions for code in the .text section. Any other section
> will cause the exception table to not be sorted correctly, and the
> exceptions will fail.
> +
> +Things changed when 64-bit support was added to x86 Linux. Rather than
> +double the size of the exception table by expanding the two entries
> +from 32-bits to 64 bits, a clever trick was used to store addreesses
s/addreesses/addresses/
> +as relative offsets from the table itself. The assembly code changed
> +from:
> + .long 1b,3b
> +to:
> + .long (from) - .
> + .long (to) - .
\n here
> +and the C-code that uses these values converts back to absolute addresses
> +like this:
<-- and here
> + ex_insn_addr(const struct exception_table_entry *x)
> + {
> + return (unsigned long)&x->insn + x->insn;
> + }
> +
> +In v4.5 the exception table entry was given a new field "handler".
> +This is also 32-bits wide and contains a third relative function
> +pointer which points to one of:
> +
> +1) int ex_handler_default(const struct exception_table_entry *fixup,
^
closing brace ------------------|
> + This is legacy case that just jumps to the fixup code
> +2) int ex_handler_fault(const struct exception_table_entry *fixup,
Ditto.
> + This case provides the fault number of the trap that occurred at
> + entry->insn. It is used to distinguish page faults from machine
> + check.
> +3) int ex_handler_ext(const struct exception_table_entry *fixup,
Ditto.
> + This case is used to for uaccess_err ... we need to set a flag
s/to //
> + in the task structure. Before the handler functions existed this
> + case was handled by adding a large offset to the fixup to tag
> + it as special.
> +More functions can easily be added.
> diff --git a/arch/x86/include/asm/asm.h b/arch/x86/include/asm/asm.h
> index 189679aba703..f5063b6659eb 100644
> --- a/arch/x86/include/asm/asm.h
> +++ b/arch/x86/include/asm/asm.h
> @@ -44,19 +44,22 @@
>
> /* Exception table entry */
> #ifdef __ASSEMBLY__
> -# define _ASM_EXTABLE(from,to) \
> +# define _ASM_EXTABLE_HANDLE(from, to, handler) \
> .pushsection "__ex_table","a" ; \
> - .balign 8 ; \
> + .balign 4 ; \
> .long (from) - . ; \
> .long (to) - . ; \
> + .long (handler) - . ; \
> .popsection
>
> -# define _ASM_EXTABLE_EX(from,to) \
> - .pushsection "__ex_table","a" ; \
> - .balign 8 ; \
> - .long (from) - . ; \
> - .long (to) - . + 0x7ffffff0 ; \
> - .popsection
> +# define _ASM_EXTABLE(from, to) \
> + _ASM_EXTABLE_HANDLE(from, to, ex_handler_default)
> +
> +# define _ASM_EXTABLE_FAULT(from, to) \
> + _ASM_EXTABLE_HANDLE(from, to, ex_handler_fault)
> +
> +# define _ASM_EXTABLE_EX(from, to) \
> + _ASM_EXTABLE_HANDLE(from, to, ex_handler_ext)
>
> # define _ASM_NOKPROBE(entry) \
> .pushsection "_kprobe_blacklist","aw" ; \
> @@ -89,19 +92,24 @@
> .endm
>
> #else
> -# define _ASM_EXTABLE(from,to) \
> +# define _EXPAND_EXTABLE_HANDLE(x) #x
> +# define _ASM_EXTABLE_HANDLE(from, to, handler) \
> " .pushsection \"__ex_table\",\"a\"\n" \
> - " .balign 8\n" \
> + " .balign 4\n" \
> " .long (" #from ") - .\n" \
> " .long (" #to ") - .\n" \
> + " .long (" _EXPAND_EXTABLE_HANDLE(handler) ") - .\n" \
> " .popsection\n"
>
> -# define _ASM_EXTABLE_EX(from,to) \
> - " .pushsection \"__ex_table\",\"a\"\n" \
> - " .balign 8\n" \
> - " .long (" #from ") - .\n" \
> - " .long (" #to ") - . + 0x7ffffff0\n" \
> - " .popsection\n"
> +# define _ASM_EXTABLE(from, to) \
> + _ASM_EXTABLE_HANDLE(from, to, ex_handler_default)
> +
> +# define _ASM_EXTABLE_FAULT(from, to) \
> + _ASM_EXTABLE_HANDLE(from, to, ex_handler_fault)
> +
> +# define _ASM_EXTABLE_EX(from, to) \
> + _ASM_EXTABLE_HANDLE(from, to, ex_handler_ext)
> +
> /* For C file, we already have NOKPROBE_SYMBOL macro */
> #endif
>
> diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
> index a4a30e4b2d34..cbcc3b3e034c 100644
> --- a/arch/x86/include/asm/uaccess.h
> +++ b/arch/x86/include/asm/uaccess.h
> @@ -90,12 +90,11 @@ static inline bool __chk_range_not_ok(unsigned long addr, unsigned long size, un
> likely(!__range_not_ok(addr, size, user_addr_max()))
>
> /*
> - * The exception table consists of pairs of addresses relative to the
> - * exception table enty itself: the first is the address of an
> - * instruction that is allowed to fault, and the second is the address
> - * at which the program should continue. No registers are modified,
> - * so it is entirely up to the continuation code to figure out what to
> - * do.
> + * The exception table consists of triples of addresses relative to the
> + * exception table enty itself. The first address is of an instruction
s/enty/entry/
> + * that is allowed to fault, the second is the target at which the program
> + * should continue. The third is a handler function to deal with the fault
> + * referenced by the instruction in the first field.a
s/referenced/caused/ i better, methinks. And the "a" at the end can go - I do
those too, btw. Lemme guess: vim user?
:-)
The rest looks good.
--
Regards/Gruss,
Boris.
ECO tip #101: Trim your mails when you reply.
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