The AMD64 Application Binary Interface specifies that the 128-byte
area below register %rsp (the 64-bit stack pointer) is considered
reserved and may not be used by signal or interrupt handlers. This is
described on page 16, section 3.2.2, and in figure 3.3:
http://www.x86-64.org/documentation.html
However in section A.2.2 on page 124, it says that the Linux kernel
doesn't honor the red zone, and so one must give GCC the -mno-red-zone
flag.
What about Mac OS X? Does OS X kernel code need -mno-red-zone? If
the red zone is to be enabled, could some of Apple's kernel code be
failing to honor it?
I've been investigating a kernel panic that has only my driver and
10.6.1 - I have installed no other software on it. If I do a lot of
I/O, I will eventually get a kernel panic. A local variable in one of
my functions will get clobbered - and always the same way. That is,
if it is clobbered at all, the value that is written into it is always
the same value.
I had a vague idea there might be a compiler bug, and indeed I found
that altering this function in very innocuous ways would make my panic
go away. But it wasn't making sense to me that it could be a compiler
bug, as it only happens after gigabytes of I/O activity. If it were a
compiler bug it ought to happen sooner.
It was while studying the disassembly of my function, and reading the
AMD64 ABI that I discovered the red zone. It turns out that my
original code uses the red zone, but if I add any subroutine calls at
all to it, it stops using the red zone - and stops panicking.
I put my code back the way it was to start with and then built my
driver with Other C++ Flags set to -mno-red-zone. I have a great big
copy that's been running for about ten minutes now and I'm not getting
the panic.
The GCC man page that comes with Xcode 3.2 says that -mkernel will set
-mno-red-zone internally. But possibly that is only for the Linux
version of GCC. What does Apple's GCC do? If -mkernel should enable
-mno-red-zone but doesn't, I'll be happy to file a radar bug.
Thanks,
Mike
--
Michael David Crawford
mdcrawford at gmail dot com
GoingWare's Bag of Programming Tricks
http://www.goingware.com/tips/
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