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x86/mm: Expand the exception table logic to allow new handling options

Browse source 
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 <[email protected]>
Reviewed-by: Borislav Petkov <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Link: http://lkml.kernel.org/r/f6af78fcbd348cf4939875cfda9c19689b5e50b8.1455732970.git.tony.luck@intel.com
Signed-off-by: Ingo Molnar <[email protected]>
This commit is contained in:
Tony Luck 2016-02-17 10:20:12 -08:00 • committed by Ingo Molnar
commit 548acf1923
8 changed files with 176 additions and 57 deletions
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35
Documentation/x86/exception-tables.txt
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@ -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.