From mboxrd@z Thu Jan 1 00:00:00 1970 From: will.deacon@arm.com (Will Deacon) Date: Wed, 2 Jul 2014 17:27:26 +0100 Subject: [PATCH v7 8/9] ARM: vdso initialization, mapping, and synchronization In-Reply-To: <53B430F3.9070804@mentor.com> References: <1403493118-7597-1-git-send-email-nathan_lynch@mentor.com> <1403493118-7597-9-git-send-email-nathan_lynch@mentor.com> <53B1D8AC.7060104@mit.edu> <20140701090309.GC28164@arm.com> <53B2C178.30607@mentor.com> <20140701141541.GP28164@arm.com> <20140702144050.GD24879@arm.com> <53B430F3.9070804@mentor.com> Message-ID: <20140702162726.GG24879@arm.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Wed, Jul 02, 2014 at 05:18:59PM +0100, Nathan Lynch wrote: > On 07/02/2014 10:54 AM, Andy Lutomirski wrote: > > Caveat 2: (major) I'm kind of surprised that this, or the current > > code, works reliably. You're doing something that I tried briefly for > > x86_64: > > > > _end = .; > > PROVIDE(end = .); > > > > . = ALIGN(PAGE_SIZE); > > PROVIDE(_vdso_data = .); > > > > This sounds great, except that you're assuming that vdso_end - > > vdso_start == ALIGN(_end, PAGE_SIZE) - (vdso base address). > > > > If you *fully* strip the vdso (eu-strip --strip-sections), then this > > is true: eu-strip --strip-sections outputs just the PT_LOAD piece of > > the vdso. But any binutils-generated incompletely stripped ELF image > > contains a section table and possible non-allocatable sections at the > > end. If these exceed the amount of unused space in the last PT_LOAD > > page, then they'll spill into the next page, and _vdso_data in the > > vdso will no longer match the address at which vdso.c loads it. Boom! > > > > I bet you're getting away with this because the whole arm64 vdso seems > > to be written in assembly, so it seems extremely unlikely to exceed > > one page minus a few hundred bytes. But if you start adding > > complexity, you might get unlucky. > > This is why I switched (in v5) the proposed 32-bit ARM VDSO to place the > data page before the code -- adding -frecord-gcc-switches to the > compiler flags was enough to break it. > > I meant to call Will's attention to it at the time for arm64's sake, but > I guess it slipped my mind... sorry. Hmm, so I could definitely look at doing the same thing, but I don't know if we actually need to for arm64. As Andy points out, we're written entirely in assembly and we objcopy -S to create the vdso.so. I've dumped the headers below and everything appears to be PT_LOAD. Will --->8 arch/arm64/kernel/vdso/vdso.so: file format elf64-littleaarch64 arch/arm64/kernel/vdso/vdso.so architecture: aarch64, flags 0x00000150: HAS_SYMS, DYNAMIC, D_PAGED start address 0x00000000000002d0 Program Header: LOAD off 0x0000000000000000 vaddr 0x0000000000000000 paddr 0x0000000000000000 align 2**4 filesz 0x00000000000006e8 memsz 0x00000000000006e8 flags r-x DYNAMIC off 0x00000000000005d8 vaddr 0x00000000000005d8 paddr 0x00000000000005d8 align 2**3 filesz 0x00000000000000f0 memsz 0x00000000000000f0 flags r-- NOTE off 0x00000000000002b8 vaddr 0x00000000000002b8 paddr 0x00000000000002b8 align 2**2 filesz 0x0000000000000018 memsz 0x0000000000000018 flags r-- EH_FRAME off 0x00000000000004f0 vaddr 0x00000000000004f0 paddr 0x00000000000004f0 align 2**2 filesz 0x0000000000000034 memsz 0x0000000000000034 flags r-- Dynamic Section: SONAME linux-vdso.so.1 HASH 0x0000000000000120 STRTAB 0x00000000000001f8 SYMTAB 0x0000000000000150 STRSZ 0x0000000000000077 SYMENT 0x0000000000000018 VERDEF 0x0000000000000280 VERDEFNUM 0x0000000000000002 VERSYM 0x0000000000000270 Version definitions: 1 0x01 0x0deebfa1 linux-vdso.so.1 2 0x00 0x075fcb89 LINUX_2.6.39 private flags = 0: Sections: Idx Name Size VMA LMA File off Algn 0 .hash 00000030 0000000000000120 0000000000000120 00000120 2**3 CONTENTS, ALLOC, LOAD, READONLY, DATA 1 .dynsym 000000a8 0000000000000150 0000000000000150 00000150 2**3 CONTENTS, ALLOC, LOAD, READONLY, DATA 2 .dynstr 00000077 00000000000001f8 00000000000001f8 000001f8 2**0 CONTENTS, ALLOC, LOAD, READONLY, DATA 3 .gnu.version 0000000e 0000000000000270 0000000000000270 00000270 2**1 CONTENTS, ALLOC, LOAD, READONLY, DATA 4 .gnu.version_d 00000038 0000000000000280 0000000000000280 00000280 2**3 CONTENTS, ALLOC, LOAD, READONLY, DATA 5 .note 00000018 00000000000002b8 00000000000002b8 000002b8 2**2 CONTENTS, ALLOC, LOAD, READONLY, DATA, LINK_ONCE_SAME_CONTENTS 6 .text 00000220 00000000000002d0 00000000000002d0 000002d0 2**4 CONTENTS, ALLOC, LOAD, READONLY, CODE 7 .eh_frame_hdr 00000034 00000000000004f0 00000000000004f0 000004f0 2**2 CONTENTS, ALLOC, LOAD, READONLY, DATA 8 .eh_frame 000000b0 0000000000000528 0000000000000528 00000528 2**3 CONTENTS, ALLOC, LOAD, READONLY, DATA 9 .dynamic 000000f0 00000000000005d8 00000000000005d8 000005d8 2**3 CONTENTS, ALLOC, LOAD, DATA 10 .got 00000008 00000000000006c8 00000000000006c8 000006c8 2**3 CONTENTS, ALLOC, LOAD, DATA 11 .got.plt 00000018 00000000000006d0 00000000000006d0 000006d0 2**3 CONTENTS, ALLOC, LOAD, DATA SYMBOL TABLE: no symbols