Hoi Ard, El Wed, May 10, 2017 at 08:51:44AM +0100 Ard Biesheuvel ha dit: > On 9 May 2017 at 22:49, Matthias Kaehlcke <mka@xxxxxxxxxxxx> wrote: > > El Tue, May 09, 2017 at 01:50:36PM -0700 Greg Hackmann ha dit: > > > >> On 05/09/2017 12:36 PM, Matthias Kaehlcke wrote: > >> >From: Greg Hackmann <ghackmann@xxxxxxxxxx> > >> > > >> >Without any extra guidance, clang will generate libstub with either > >> >absolute or relative ELF relocations. Use the right combination of > >> >-fpic and -fno-pic on different files to avoid this. > >> > > >> >Signed-off-by: Greg Hackmann <ghackmann@xxxxxxxxxx> > >> >Signed-off-by: Bernhard Rosenkränzer <Bernhard.Rosenkranzer@xxxxxxxxxx> > >> >Signed-off-by: Matthias Kaehlcke <mka@xxxxxxxxxxxx> > >> >--- > >> > drivers/firmware/efi/libstub/Makefile | 6 ++++++ > >> > 1 file changed, 6 insertions(+) > >> > > >> >diff --git a/drivers/firmware/efi/libstub/Makefile b/drivers/firmware/efi/libstub/Makefile > >> >index f7425960f6a5..ccbaaf4d8650 100644 > >> >--- a/drivers/firmware/efi/libstub/Makefile > >> >+++ b/drivers/firmware/efi/libstub/Makefile > >> >@@ -11,6 +11,9 @@ cflags-$(CONFIG_X86) += -m$(BITS) -D__KERNEL__ -O2 \ > >> > -mno-mmx -mno-sse > >> > > >> > cflags-$(CONFIG_ARM64) := $(subst -pg,,$(KBUILD_CFLAGS)) > >> >+ifeq ($(cc-name),clang) > >> >+cflags-$(CONFIG_ARM64) += -fpic > >> >+endif > >> > cflags-$(CONFIG_ARM) := $(subst -pg,,$(KBUILD_CFLAGS)) \ > >> > -fno-builtin -fpic -mno-single-pic-base > >> > > >> >@@ -38,6 +41,9 @@ $(obj)/lib-%.o: $(srctree)/lib/%.c FORCE > >> > > >> > lib-$(CONFIG_EFI_ARMSTUB) += arm-stub.o fdt.o string.o random.o \ > >> > $(patsubst %.c,lib-%.o,$(arm-deps)) > >> >+ifeq ($(cc-name),clang) > >> >+CFLAGS_arm64-stub.o += -fno-pic > >> >+endif > >> > > >> > lib-$(CONFIG_ARM) += arm32-stub.o > >> > lib-$(CONFIG_ARM64) += arm64-stub.o > >> > > >> > >> NAK. > >> > >> This patch was labeled "HACK:" in our experimental tree. There's no > >> rhyme or reason to why this combination of -f[no-]pic flags > >> generates code without problematic relocations. It's inherently > >> fragile, and was only intended as a temporary workaround until I (or > >> someone more familiar with EFI) got a chance to revisit the problem. > >> > >> Unless the gcc CFLAGS are also an artifact of "mess with -f[no-]pic > >> until the compiler generates what you want", this doesn't belong > >> upstream. > > > > Sorry, I didn't realize it is that bad of a hack. Unfortunately I'm > > not very familiar with EFI either. > > > > I saw Ard did some work in this code related with relocation, maybe he > > can provide a pointer towards a better solution. > > > > This is a known issue. The problem is that generic AArch64 small model > code is mostly position independent already, due to its use of > adrp/add pairs to generate symbol references with a +/- 4 GB range. > Building the same code with -fpic will result in GOT entries to be > generated, which carry absolute addresses, so this achieves the exact > opposite of what we want. > > The reason for the GOT entries is that GCC (and Clang, apparently) > infer from the -fpic flag that you are building objects that will be > linked into a shared library, to which ELF symbol preemption rules > apply that stipulate that a symbol in the main executable supersedes a > symbol under the same name in the shared library, and that the shared > library should update all its internal references to the main > executable's version of the symbol. The easiest way (but certainly not > the only way) to achieve that is to indirect all internal symbol > references via GOT entries, which can be made to refer to another > symbol by updating a single value. > > The workaround I used is to use hidden visibility, using a #pragma. > (There is a -fvisibility=hidden command line option as well, but this > is a weaker form that does not apply to extern declarations, only to > definitions). So if you add > > #pragma GCC visibility push(hidden) > > at the beginning of arm64-stub.c (and perhaps to one or two other > files that contain externally visible symbol declarations these days), > you should be able to compile the entire EFI stub with -fpic. Note > that making those externally visible symbols 'static' where possible > would solve the problem as well, but this triggers another issue in > the 32-bit ARM stub. > > In my opinion, the correct fix would be to make -fpie (as opposed to > -fpic) imply hidden visibility, given that PIE executables don't > export symbols in the first place, and so the preemption rules do not > apply. It is worth a try whether -fpie works as expected in this case > on Clang, but the last time I tried it on GCC, it behaved exactly like > -fpic. Thanks a lot for the detailed description and your suggestions! A clang build with -fpie for the EFI stub succeeds without complaints about GOT entries. I will send out an updated patch (with -fpie only for clang) later. Cheers Matthias -- To unsubscribe from this list: send the line "unsubscribe linux-efi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
