Date: Wed, 26 Sep 2001 17:20:31 +0100 From: Nick Barnes <Nick.Barnes@pobox.com> To: freebsd-hackers@freebsd.org Subject: distinguising read faults and write faults Message-ID: <52872.1001521231@thrush.ravenbrook.com>
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I am in the process of porting an incremental garbage collector to
FreeBSD on x86. This garbage collector uses read barriers and write
barriers for incrementality. The details are irrelevant; the key part
is that I want to protect parts of memory and handle faults on the
protected pages.
On FreeBSD, I will be using mmap(MAP_ANON) to obtain memory, and
mprotect() to raise the barriers, with a signal handler to handle
barrier hits.
I have worked out that I need to handle SIGBUS (not SIGSEGV, which
surprised me), and that ucontext->mcontext.mc_trapno will be 12
(T_PAGEFLT, not T_PROTFLT, which surprised me). However, I don't see
any way of distinguishing between read faults and write faults. This
information doesn't appear to be anywhere in the ucontext or the
siginfo.
Is there any way of distingushing read faults from write faults (other
than by decoding the instruction at mc_eip)?
Matching code for Linux below:
Nick Barnes
/* distinguishing read faults from write faults on Linux */
static void sigHandle(int sig, struct sigcontext context)
{
if(context.trapno == 14) { /* page fault */
int write = ((context.err & 0x2) != 0); /* write */
...
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