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From: David Howells <dhowells@redhat.com>
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To: Andrew Pinski <pinskia@gmail.com>
Cc: dhowells@redhat.com, "H. Peter Anvin" <hpa@zytor.com>,
    linux-kernel@vger.kernel.org
Subject: Re: [PATCH 00/45] C++: Convert the kernel to C++
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Date: Wed, 10 Jan 2024 00:29:24 +0000
Message-ID: <1682637.1704846564@warthog.procyon.org.uk>

Andrew Pinski <pinskia@gmail.com> wrote:

> Note even in GCC, we disable exceptions and RTTI while building GCC.
> This is specifically due to not wanting to use them and use other
> methods to do that.
> Note GDB on the other hand used to use setjmp/longjmp for their
> exception handling in C and I think they moved over to using C++
> exceptions which simplified things there. But as far as I know the
> Linux kernel does not use a mechanism like that (I know of copy
> from/to user using HW exceptions/error/interrupt handling but that is
> a special case only).

If we were to allow exception handling, I wonder if we would actually need to
throw anything other than a signed long integer (e.g. an error code) and just
disable RTTI.  Maybe something like:

	long sys_rename(...)
	{
		struct rwsem_lock lock_a, lock_b;
		struct inode *dir_a, *dir_b;
		...
		try {
			if (dir_a > dir_b) {
				lock_a.down_write_killable(dir_a);
				lock_b.down_write_killable(dir_b);
			} else {
				lock_b.down_write_killable(dir_b);
				lock_a.down_write_killable(dir_a);
			}
		} catch (-EINTR) {
			throw -ERESTARTSYS;
		}
		...
	}

then have a cut-down exception unwinder that only needs to deal with long
values.

However, I think rolling out exception handling in the kernel might be too big
a task, given the huge amount of code involved - however much we might like to
avoid all those return value checks.

David