El Thu, May 11, 2017 at 02:51:48PM +0100 Ard Biesheuvel ha dit: > (adding Arnd and Will to cc, who are likely to have an opinion as to > which GCC is the oldest we need to support for arm64) > > On 10 May 2017 at 20:47, Matthias Kaehlcke <mka@xxxxxxxxxxxx> wrote: > > El Wed, May 10, 2017 at 09:05:28PM +0200 Ard Biesheuvel ha dit: > > > >> > >> > >> > On 10 May 2017, at 20:38, Matthias Kaehlcke <mka@xxxxxxxxxxxx> wrote: > >> > > >> > 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. > >> > > >> > >> Good! I never liked the visibility hack, which is why I never upstreamed it. > >> > >> Could you please check how recent GCC behaves? > > > > I tried GCC v4.9.4 and v6.3.1, both build the EFI stub with -fpie > > without errors. > > > > Are you suggesting to use -fpie for both clang and GCC? Do you know > > what the minimum required GCC version is for building an arm64 kernel? > > Yes. Up until now, we have been relying on the position independent > nature of small model code, but it would be better to specify it > explicitly, so if -fpie gives us mostly identical code and does not > need visibility hacks, I would prefer to add it for all compilers and > not have an exception only for Clang. Note that the same applies to > the entire kernel when built in KASLR mode, so it would also be good > to know our options here. Thanks, makes perfect sense. > Arnd, Will, what is the oldest GCC version we claim to support for arm64? 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