On Wed, Jul 17, 2019 at 5:02 PM Vaibhav Rustagi <vaibhavrustagi@xxxxxxxxxx> wrote: > > From: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> > > Implementing memcpy and memset in terms of __builtin_memcpy and > __builtin_memset is problematic. > > GCC at -O2 will replace calls to the builtins with calls to memcpy and > memset (but will generate an inline implementation at -Os). Clang will > replace the builtins with these calls regardless of optimization level. > > $ llvm-objdump -dr arch/x86/purgatory/string.o | tail > > 0000000000000339 memcpy: > 339: 48 b8 00 00 00 00 00 00 00 00 movabsq $0, %rax > 000000000000033b: R_X86_64_64 memcpy > 343: ff e0 jmpq *%rax > > 0000000000000345 memset: > 345: 48 b8 00 00 00 00 00 00 00 00 movabsq $0, %rax > 0000000000000347: R_X86_64_64 memset > 34f: ff e0 > > Such code results in infinite recursion at runtime. This is observed > when doing kexec. Just so it's crystal clear to other reviewers, consider this codegen between compilers and optimization levels: https://godbolt.org/z/jcfKsw So I'd imagine the commit that introduced these implementations very much relied on being compiled at -Os to work. > > Instead, reuse an implementation from arch/x86/boot/compressed/string.c > if we define warn as a symbol. Alternatively, I was getting fancy trying to match what GCC lowers __builtin_memcpy/__builtin_memset to: diff --git a/arch/x86/purgatory/string.c b/arch/x86/purgatory/string.c index 795ca4f..e055f65 100644 --- a/arch/x86/purgatory/string.c +++ b/arch/x86/purgatory/string.c @@ -16,10 +16,23 @@ void *memcpy(void *dst, const void *src, size_t len) { - return __builtin_memcpy(dst, src, len); + asm( + "movq %0, %%rax\n\t" + "movq %2, %%rcx\n\t" + "rep movsb\n\t" + : "=r"(dst) : "r"(src), "ri"(len) : "rax", "rcx"); + return dst; } void *memset(void *dst, int c, size_t len) { - return __builtin_memset(dst, c, len); + void* ret; + asm( + "movq %1, %%r8\n\t" + "movl %2, %%eax\n\t" + "movq %3, %%rcx\n\t" + "rep stosb\n\t" + "movq %%r8, %0" + : "=r"(ret) : "r"(dst), "ri"(c), "ri"(len) : "r8", "eax", "rcx"); + return ret; } but then Alistair pointed out that we have a proliferation of memcpy+memest definitions in the kernel, and we should probably just reuse an existing one rather than add to the arms race. > > Link: https://bugs.chromium.org/p/chromium/issues/detail?id=984056 > Reported-by: Vaibhav Rustagi <vaibhavrustagi@xxxxxxxxxx> > Tested-by: Vaibhav Rustagi <vaibhavrustagi@xxxxxxxxxx> > Debugged-by: Vaibhav Rustagi <vaibhavrustagi@xxxxxxxxxx> > Debugged-by: Manoj Gupta <manojgupta@xxxxxxxxxx> > Suggested-by: Alistair Delva <adelva@xxxxxxxxxx> > Signed-off-by: Vaibhav Rustagi <vaibhavrustagi@xxxxxxxxxx> > Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> > --- > arch/x86/purgatory/Makefile | 3 +++ > arch/x86/purgatory/purgatory.c | 6 ++++++ > arch/x86/purgatory/string.c | 23 ----------------------- > 3 files changed, 9 insertions(+), 23 deletions(-) > delete mode 100644 arch/x86/purgatory/string.c > > diff --git a/arch/x86/purgatory/Makefile b/arch/x86/purgatory/Makefile > index 3589ec4a28c7..84b8314ddb2d 100644 > --- a/arch/x86/purgatory/Makefile > +++ b/arch/x86/purgatory/Makefile > @@ -6,6 +6,9 @@ purgatory-y := purgatory.o stack.o setup-x86_$(BITS).o sha256.o entry64.o string > targets += $(purgatory-y) > PURGATORY_OBJS = $(addprefix $(obj)/,$(purgatory-y)) > > +$(obj)/string.o: $(srctree)/arch/x86/boot/compressed/string.c FORCE > + $(call if_changed_rule,cc_o_c) > + > $(obj)/sha256.o: $(srctree)/lib/sha256.c FORCE > $(call if_changed_rule,cc_o_c) > > diff --git a/arch/x86/purgatory/purgatory.c b/arch/x86/purgatory/purgatory.c > index 6d8d5a34c377..b607bda786f6 100644 > --- a/arch/x86/purgatory/purgatory.c > +++ b/arch/x86/purgatory/purgatory.c > @@ -68,3 +68,9 @@ void purgatory(void) > } > copy_backup_region(); > } > + > +/* > + * Defined in order to reuse memcpy() and memset() from > + * arch/x86/boot/compressed/string.c > + */ > +void warn(const char *msg) {} This is the one part I feel bad about; memcpy() in arch/x86/boot/compressed/string.c calls warn() which would result in an undefined symbol in purgatory.ro. Maybe there's a preferred solution, or this is ok for purgatory/kexec? There's other x86 memsets+memcpys, but IMO this is the smallest incision without playing the satisfy-the-symbol-dependencies game. If the maintainers are ok with this, then the series looks ready to go to me. Thanks for debugging/sending Vaibhav. Orthogonally, I showed Hans Boehm the pointer comparisons+subtraction in arch/x86/boot/compressed/string.c's memcpy asking about pointer provenance issues (https://wiki.sei.cmu.edu/confluence/display/c/ARR36-C.+Do+not+subtract+or+compare+two+pointers+that+do+not+refer+to+the+same+array, http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2090.htm) introduced in commit 00ec2c37031e ("x86/boot: Warn on future overlapping memcpy() use") and he started cursing in Spanish (I don't think he speaks Spanish) and performed the sign of the cross. Y'all need <strikethrough>Jesus</strikethrough>[u]intptr_t. > diff --git a/arch/x86/purgatory/string.c b/arch/x86/purgatory/string.c > deleted file mode 100644 > index 01ad43873ad9..000000000000 > --- a/arch/x86/purgatory/string.c > +++ /dev/null > @@ -1,23 +0,0 @@ > -// SPDX-License-Identifier: GPL-2.0-only > -/* > - * Simple string functions. > - * > - * Copyright (C) 2014 Red Hat Inc. > - * > - * Author: > - * Vivek Goyal <vgoyal@xxxxxxxxxx> > - */ > - > -#include <linux/types.h> > - > -#include "../boot/string.c" > - > -void *memcpy(void *dst, const void *src, size_t len) > -{ > - return __builtin_memcpy(dst, src, len); > -} > - > -void *memset(void *dst, int c, size_t len) > -{ > - return __builtin_memset(dst, c, len); > -} > -- > 2.22.0.510.g264f2c817a-goog > -- Thanks, ~Nick Desaulniers
