Date: Thu, 14 Nov 2024 09:47:30 -0500 From: John Baldwin <jhb@FreeBSD.org> To: Jessica Clarke <jrtc27@freebsd.org> Cc: Andrew Turner <andrew@FreeBSD.org>, "src-committers@freebsd.org" <src-committers@FreeBSD.org>, "dev-commits-src-all@freebsd.org" <dev-commits-src-all@FreeBSD.org>, "dev-commits-src-main@freebsd.org" <dev-commits-src-main@FreeBSD.org> Subject: Re: git: e85eaa930862 - main - Have rtld query the page size from the kernel Message-ID: <4d81d34f-4749-4911-b302-eca9166e4be7@FreeBSD.org> In-Reply-To: <768D45F9-2F02-4BA1-BFB7-51685486CFCC@freebsd.org> References: <202204071438.237Ecn2A012737@gitrepo.freebsd.org> <92a05dfe-683c-43d9-bd29-3110e89be275@FreeBSD.org> <768D45F9-2F02-4BA1-BFB7-51685486CFCC@freebsd.org>
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On 11/13/24 12:10, Jessica Clarke wrote: > On 13 Nov 2024, at 19:44, John Baldwin <jhb@FreeBSD.org> wrote: >> >> On 4/7/22 07:38, Andrew Turner wrote: >>> The branch main has been updated by andrew: >>> URL: https://cgit.FreeBSD.org/src/commit/?id=e85eaa930862d5b4dc917bc31e8d7254a693635d >>> commit e85eaa930862d5b4dc917bc31e8d7254a693635d >>> Author: Andrew Turner <andrew@FreeBSD.org> >>> AuthorDate: 2022-04-04 15:05:40 +0000 >>> Commit: Andrew Turner <andrew@FreeBSD.org> >>> CommitDate: 2022-04-07 14:37:37 +0000 >>> Have rtld query the page size from the kernel >>> To allow for a dynamic page size on arm64 have the runtime linker >>> query the kernel for the currentl page size. >>> Reviewed by: kib >>> Sponsored by: The FreeBSD Foundation >>> Differential Revision: https://reviews.freebsd.org/D34765 >> >> This broke relro handling for rtld. The reason is that init_pagesizes() is >> called after parsing the program headers for rltd in init_rtld(). As a result, >> page_size is 0 when rtld_round_page() is called so the relro_size is 0. The >> RTLD_INIT_EARLY_PAGESIZES case was for ia64, and in the early case it's probably >> not safe to call sysctl? If it is safe to call sysctl, we could just always >> init pagesizes early? > > It looks like there are a few things going on: > > 1. relocate_object calls obj_enforce_relro if !obj->mainprog, so will > try to enforce RELRO for RTLD itself whilst page_size is 0 > > 2. init_rtld later calls obj_enforce_relro for obj_rtld, after > page_size has been initialised > > 3. init_rtld is careful to avoid using global variables until it’s > called relocate_objects for RTLD itself, but by hiding accesses to > page_size away in rtld_*_page that’s no longer true (definitely not > true in the case of text relocations, for example, though whether it > also occurs for other cases we care more about I don’t know) > > So I think there are a couple of things to fix: > > 1. Stop accessing page_size prior to relocate_objects returning for > RTLD itself > > 2. Stop enforcing RELRO twice for RTLD (e.g. add && obj != rtldobj to > relocate_object’s case) > > At least, that’s what I’ve inferred from reading the code. > > Though, to be honest, things might be rather nicer if we just made > .rtld_start responsible for relocating RTLD itself prior to calling > init_rtld, that’s what we have to do for CHERI, as do arm, powerpc and > powerpc64, and it means you can use globals from the start in init_rtld. I've done 2) locally which fixed my immediate issue. I agree though that having all arches implement rtld_relocate_nonplt_self() which is called before rtld() would be nicer. -- John Baldwin
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