Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758747AbZFIH1A (ORCPT ); Tue, 9 Jun 2009 03:27:00 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1756697AbZFIH0x (ORCPT ); Tue, 9 Jun 2009 03:26:53 -0400 Received: from cantor2.suse.de ([195.135.220.15]:59383 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755834AbZFIH0w (ORCPT ); Tue, 9 Jun 2009 03:26:52 -0400 Subject: Re: [PATCH] x86: clean up vdso-layout.lds.S From: Petr Tesarik To: Sam Ravnborg Cc: LKML , Thomas Gleixner , Ingo Molnar , "H. Peter Anvin" , Andi Kleen , Roland McGrath In-Reply-To: <20090605200701.GB23195@uranus.ravnborg.org> References: <1243865115.24278.8.camel@nathan.suse.cz> <1244215559.1604.12.camel@nathan.suse.cz> <20090605200701.GB23195@uranus.ravnborg.org> Content-Type: text/plain; charset="UTF-8" Organization: SUSE LINUX Date: Tue, 09 Jun 2009 09:26:58 +0200 Message-Id: <1244532418.5199.24.camel@nathan.suse.cz> Mime-Version: 1.0 X-Mailer: Evolution 2.24.1.1 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 6061 Lines: 169 Sam Ravnborg píše v Pá 05. 06. 2009 v 22:07 +0200: > > > diff --git a/arch/x86/vdso/Makefile b/arch/x86/vdso/Makefile > > > index 16a9020..8c7f06a 100644 > > > --- a/arch/x86/vdso/Makefile > > > +++ b/arch/x86/vdso/Makefile > > > @@ -23,7 +23,8 @@ $(obj)/vdso.o: $(obj)/vdso.so > > > > > > targets += vdso.so vdso.so.dbg vdso.lds $(vobjs-y) > > > > > > -export CPPFLAGS_vdso.lds += -P -C > > > +vdso-cppflags = -P -C > > > +export CPPFLAGS_vdso.lds += -m64 $(vdso-cppflags) > > I am wondering why we need -P -C here - but we do not need it for lds.S files? > Seems like something we could let go. Frankly, I don't know, but it's been there for ages, and I don't see what you mean, anyway. Doesn't the top-level Makefile contain this line, for example: export CPPFLAGS_vmlinux.lds += -P -C -U$(ARCH) > > > VDSO_LDFLAGS_vdso.lds = -m elf_x86_64 -Wl,-soname=linux-vdso.so.1 \ > > > -Wl,-z,max-page-size=4096 -Wl,-z,common-page-size=4096 > > > @@ -68,7 +69,7 @@ vdso32.so-$(VDSO32-y) += sysenter > > > > > > vdso32-images = $(vdso32.so-y:%=vdso32-%.so) > > > > > > -CPPFLAGS_vdso32.lds = $(CPPFLAGS_vdso.lds) > > > +CPPFLAGS_vdso32.lds = -m32 $(vdso-cppflags) > > > VDSO_LDFLAGS_vdso32.lds = -m elf_i386 -Wl,-soname=linux-gate.so.1 > > > > > > # This makes sure the $(obj) subdirectory exists even though vdso32/ > > > diff --git a/arch/x86/vdso/vdso-layout.lds.S b/arch/x86/vdso/vdso-layout.lds.S > > > index 634a2cf..1f4b215 100644 > > > --- a/arch/x86/vdso/vdso-layout.lds.S > > > +++ b/arch/x86/vdso/vdso-layout.lds.S > > > @@ -22,16 +22,15 @@ SECTIONS > > > .eh_frame : { KEEP (*(.eh_frame)) } :text > > > > > > .dynamic : { *(.dynamic) } :text :dynamic > > > + .got : { *(.got.plt) *(.got) } :text > > The style we try to introduce for .lds files in > arch/$ARCH/kernel/vmlinux.lds.S is much more C-like. > The above would have been: > .got : { > *(.got.plt) > *(.got) > } :text > > Please use this all over so we have a consistent style in linker scripts. OK, so should I first post a patch which doesn't change anything but adjusts the coding style of vdso-layout.lds.S? > > > .data : { > > > - *(.data*) > > > - *(.sdata*) > > > - *(.got.plt) *(.got) > > > - *(.gnu.linkonce.d.*) > > > - *(.bss*) > > > - *(.dynbss*) > > > - *(.gnu.linkonce.b.*) > > > + *(.data .data.* .gnu.linkonce.d.*) > > > + *(.bss .bss.* .gnu.linkonce.b.*) > > > + *(COMMON) > > > } > Where did *(.sdata*) go? Nothing changed, really. This section is never produced for i386 or x86_64. The line might have been a remnant of the (long abandoned) idea that x86_64 would have .sdata and .data, but it then turned to be rather .data and .ldata. > Why do we need *(.data .data.*) rather than *(.data*)? > *(.dynbss*)? I don't have any strong opinion here, but the former is exactly what the default linker script has. > In your changelog you say: > "discard sections which are not useful to user-space" - but as you do not > list which one it is hard to tell what you removed on purpose > and what you removed by accident. All those which end up in the section ".broken". I'll make a better wording next time. > > > > > > .altinstructions : { *(.altinstructions) } > > > @@ -43,9 +42,49 @@ SECTIONS > > > */ > > > . = ALIGN(0x100); > > What is 0x100? Um. No idea. Roland, you added this line in commit f6b46ebf904f69a73907a5e6b1ed2228e3f03d9e. Can you shed some light on this magic constant? > > > - .text : { *(.text*) } :text =0x90909090 > > > + .text : { > > > + *(.text .text.* .gnu.linkonce.t.*) > > > + } :text =0x90909090 > > > + > > > + /* We would need a more sophisticated dynamic linker for the > > > + * vDSO to make the following sections work. Put them into > > > + * a special section and raise a link-time error if they get > > > + * used. > > > + */ > > > + .broken : { > > > + /* Code in the Procedure Linkage Table will segfault */ > > > + *(.plt) > > > + > > > + /* Relocation will not be done, so any pointers will > > > + * still point to the prelinked address, which is wrong > > > + */ > > > + *(.data.rel.ro*) > > > + *(.gnu.linkonce.d.rel.ro.*) > > > + > > > + /* Initialization/termination won't work this way */ > > > + *(.init) *(.fini) > > > + *(.preinit_array) *(.init_array*) > > > + *(.fini_array*) > > > + > > > + /* Thread-local data cannot be defined like this */ > > > + *(.tdata .tdata.* .gnu.linkonce.td.*) > > > + *(.tbss .tbss.* .gnu.linkonce.tb.*) > > > + *(.tcommon) > > > + } > > > + > > > + /* These sections are not useful */ > > > + /DISCARD/ : { > > > + *(.gnu.warning.*) > > > + *(.note.GNU-stack) > > > + } > > > } > > > > > > +ASSERT(!SIZEOF(.broken), "VDSO contains sections that don't work properly"); > Can you give any better hints where to look and what to look for? What would you expect? The linker script language is quite limited in its capabilities... Best I could do is split the ".broken" section into several sections and move the descriptions from the individual comments above here. If this muckle of empty ".broken.*" sections gets correctly discarded and triggers no bug in binutils, I can probably do it. > > > + > > > +/* Check that GOT has only the three entries defined by the ABI */ > > > +ASSERT(SIZEOF(.got) == 3*__SIZEOF_POINTER__, > > > + "Found extra GOT entries. Check your use of external vars."); > Can you give any better hints where to look and what to look for? What would you consider a better hint? If there are any entries in the GOT, this means that the vDSO tries to access an external function or variable. But since we don't have a real in-kernel linker for the vDSO, these references will remain unresolved (and most likely cause a segmentation fault at run-time). Cheers, Petr Tesarik -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/