From 548acf19234dbda5a52d5a8e7e205af46e9da840 Mon Sep 17 00:00:00 2001 From: Tony Luck Date: Wed, 17 Feb 2016 10:20:12 -0800 Subject: x86/mm: Expand the exception table logic to allow new handling options 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 Reviewed-by: Borislav Petkov Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Thomas Gleixner Link: http://lkml.kernel.org/r/f6af78fcbd348cf4939875cfda9c19689b5e50b8.1455732970.git.tony.luck@intel.com Signed-off-by: Ingo Molnar --- Documentation/x86/exception-tables.txt | 35 ++++++++++++++++++++++++++++++++++ 1 file changed, 35 insertions(+) (limited to 'Documentation/x86') diff --git a/Documentation/x86/exception-tables.txt b/Documentation/x86/exception-tables.txt index 32901aa36f0a..e396bcd8d830 100644 --- a/Documentation/x86/exception-tables.txt +++ b/Documentation/x86/exception-tables.txt @@ -290,3 +290,38 @@ 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 addresses +as relative offsets from the table itself. The assembly code changed +from: + .long 1b,3b +to: + .long (from) - . + .long (to) - . + +and the C-code that uses these values converts back to absolute addresses +like this: + + ex_insn_addr(const struct exception_table_entry *x) + { + return (unsigned long)&x->insn + x->insn; + } + +In v4.6 the exception table entry was expanded with 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) + This is legacy case that just jumps to the fixup code +2) int ex_handler_fault(const struct exception_table_entry *fixup) + 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) + This case is used for uaccess_err ... we need to set a flag + 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. -- cgit