Re: [PATCH v2] random: vDSO: Avoid call to memset() when zeroing reserved in __cvdso_getrandom_data()

["Jason A. Donenfeld" <Jason@zx2c4.com>]

On Sat, Sep 26, 2026 at 12:55:34PM +0200, Christophe Leroy (CS GROUP) wrote:
> Hi Jason,
> 
> Le 26/09/2026 à 12:02, Jason A. Donenfeld a écrit :
> > On Fri, Sep 25, 2026 at 03:00:46PM -0700, Nick Desaulniers wrote:
> >> On Fri, Sep 25, 2026 at 2:56 PM Nick Desaulniers
> >> <ndesaulniers@google.com> wrote:
> >>>
> >>> On Fri, Sep 25, 2026 at 2:53 PM Nick Desaulniers
> >>> <ndesaulniers@google.com> wrote:
> >>>>
> >>>> On Fri, Sep 25, 2026 at 2:46 PM Nathan Chancellor <nathan@kernel.org> wrote:
> >>>>>
> >>>>> After a recent change in LLVM [1], builds with the random vDSO
> >>>>> implementation, such as PowerPC and RISC-V, fail when checking the vDSO:
> >>>>>
> >>>>>    arch/powerpc/kernel/vdso/vdso32.so.dbg: dynamic relocations are not supported
> >>>>>    arch/riscv/kernel/vdso/vdso.so.dbg: dynamic relocations are not supported
> >>>>>
> >>>>> memset() is now generated when zeroing params->reserved for some builds
> >>>>> because LLVM has an optimization (now run in more instances) that can
> >>>>> recognize at compile time when it is assigning a static value to a
> >>>>> contiguous area of memory and turn that into a call to memset(). Both
> >>>>> clang and GCC assume memset() is always available [2].
> >>>>>
> >>>>> clang provides a builtin, __builtin_memset_inline [3][4], that can be
> >>>>> used to ensure an external function call is not generated when zeroing
> >>>>> this memory. Use it when it is available.
> >>>>>
> >>>>> While GCC has no issues with the current code, it has generated memset()
> >>>>> before, as seen in commit b7bad082e113 ("random: vDSO: avoid call to out
> >>>>> of line memset()"). GCC 14 provides '-finline-stringops=memset' [5][6]
> >>>>> with a similar guarantee to the clang builtin, so use it and
> >>>>> __builtin_memset() when available.
> >>>>>
> >>>>> If no option is available, provide a simple memset_inline() like the one
> >>>>> from lib/string.c to avoid adding an ugly ifdef.
> >>>>>
> >>>>> Link: https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fllvm%2Fllvm-project%2Fcommit%2F90cebef1411617fc3eedd359bdf00cb44b1c2439&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7C4df96063ffd94e6204df08df1bb54e3d%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C639260137659627802%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=ku4l8Ol5OJ2qi%2BcSblspxxUKOA6rNpiUO3ZvGSoAb1s%3D&reserved=0 [1]
> >>>>> Link: https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgcc.gnu.org%2Fonlinedocs%2Fgcc-16.2.0%2Fgcc%2FStandards.html%23index-ffreestanding&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7C4df96063ffd94e6204df08df1bb54e3d%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C639260137659651194%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=jQa1eTDLk3cPNTa3GEBwVEohHh67ukShaRHMZvA%2FHDg%3D&reserved=0 [2]
> >>>>> Link: https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fllvm%2Fllvm-project%2Fcommit%2F38637ee477541370a90b37f149069d8e5c0c2efd&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7C4df96063ffd94e6204df08df1bb54e3d%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C639260137659668579%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=zY7Kr9ZZNNZR%2FVsnwYRlubPRpbMaKe9QtMD2snd8ZvA%3D&reserved=0 [3]
> >>>>> Link: https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fclang.llvm.org%2Fdocs%2FLanguageExtensions.html%23guaranteed-inlined-memset&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7C4df96063ffd94e6204df08df1bb54e3d%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C639260137659678360%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=vUU2fhCVcUWsbQD3IsRaUzSO8KVSpxwSpgr9San4aq0%3D&reserved=0 [4]
> >>>>> Link: https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgcc.gnu.org%2Fgit%2F%3Fp%3Dgcc.git%3Ba%3Dcommit%3Bh%3D1ff6d9f7428b0668cd8ab0b3e3ab94f1d733124d&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7C4df96063ffd94e6204df08df1bb54e3d%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C639260137659687815%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=qHRqxAO252imwH1W93yHlp6jdcFGkFOr2Y2hXbG2MLU%3D&reserved=0 [5]
> >>>>> Link: https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgcc.gnu.org%2Fonlinedocs%2Fgcc%2FOptimize-Options.html%23index-finline-stringops&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7C4df96063ffd94e6204df08df1bb54e3d%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C639260137659698298%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=0ecc%2BXd6o3nozh3N71aq%2F98Xam%2Fdol7GUHmC637Iv7I%3D&reserved=0 [6]
> >>>>> Suggested-by: "Jason A. Donenfeld" <Jason@zx2c4.com>
> >>>>> Suggested-by: Nick Desaulniers <ndesaulniers@google.com>
> >>>>> Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> >>>>> ---
> >>>>> Changes in v2:
> >>>>> - Switch approach entirely (Jason, Nick)
> >>>>>    - clang: use __builtin_memset_inline() to avoid external call
> >>>>>    - GCC 14+: use __builtin_memset + -finline-stringops=memset (hence the
> >>>>>      massive CC list increase)
> >>>>>    - GCC < 14: no issues currently but avoid ifdef with simple memset
> >>>>>      implementation
> >>>>> - Link to v1: https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fpatch.msgid.link%2F20260916-vdso-getrandom-avoid-memset-llvm-24-v1-1-80a92f2e225a%40kernel.org&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7C4df96063ffd94e6204df08df1bb54e3d%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C639260137659712983%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=%2Ftq3tUHqDmbXesqB69w6umXAuaTsfzOIhk54Jy76cwo%3D&reserved=0
> >>>>> ---
> >>>>>   arch/arm64/kernel/vdso/Makefile     |  2 +-
> >>>>>   arch/loongarch/vdso/Makefile        |  1 +
> >>>>>   arch/powerpc/kernel/vdso/Makefile   |  1 +
> >>>>>   arch/riscv/kernel/vdso/Makefile     |  1 +
> >>>>>   arch/s390/kernel/vdso/Makefile      |  1 +
> >>>>>   arch/x86/entry/vdso/vdso64/Makefile |  2 +-
> >>>>>   init/Kconfig                        |  7 +++++++
> >>>>>   lib/vdso/getrandom.c                | 19 +++++++++++++++++--
> >>>>>   8 files changed, 30 insertions(+), 4 deletions(-)
> >>>>>
> >>>>> diff --git a/arch/arm64/kernel/vdso/Makefile b/arch/arm64/kernel/vdso/Makefile
> >>>>> index 7dec05dd33b7..f6619e1cb2ce 100644
> >>>>> --- a/arch/arm64/kernel/vdso/Makefile
> >>>>> +++ b/arch/arm64/kernel/vdso/Makefile
> >>>>> @@ -41,7 +41,7 @@ CC_FLAGS_REMOVE_VDSO := $(CC_FLAGS_FTRACE) -Os $(CC_FLAGS_SCS) \
> >>>>>                          $(CC_FLAGS_LTO) $(CC_FLAGS_CFI) \
> >>>>>                          -Wmissing-prototypes -Wmissing-declarations
> >>>>>
> >>>>> -CC_FLAGS_ADD_VDSO := -O2 -mcmodel=tiny -fasynchronous-unwind-tables
> >>>>> +CC_FLAGS_ADD_VDSO := -O2 -mcmodel=tiny -fasynchronous-unwind-tables $(CONFIG_CC_OPT_INLINE_MEMSET)
> >>>>>
> >>>>>   CFLAGS_REMOVE_vgettimeofday.o = $(CC_FLAGS_REMOVE_VDSO)
> >>>>>   CFLAGS_REMOVE_vgetrandom.o = $(CC_FLAGS_REMOVE_VDSO)
> >>>>> diff --git a/arch/loongarch/vdso/Makefile b/arch/loongarch/vdso/Makefile
> >>>>> index 9c9181bb4071..0b84892d084b 100644
> >>>>> --- a/arch/loongarch/vdso/Makefile
> >>>>> +++ b/arch/loongarch/vdso/Makefile
> >>>>> @@ -16,6 +16,7 @@ ccflags-vdso := \
> >>>>>          $(filter -m64,$(KBUILD_CFLAGS)) \
> >>>>>          $(filter -march=%,$(KBUILD_CFLAGS)) \
> >>>>>          $(filter -m%-float,$(KBUILD_CFLAGS)) \
> >>>>> +       $(CONFIG_CC_OPT_INLINE_MEMSET) \
> >>>>>          $(CLANG_FLAGS) \
> >>>>>          -D__VDSO__
> >>>>>
> >>>>> diff --git a/arch/powerpc/kernel/vdso/Makefile b/arch/powerpc/kernel/vdso/Makefile
> >>>>> index 368759f81708..0d1a49529885 100644
> >>>>> --- a/arch/powerpc/kernel/vdso/Makefile
> >>>>> +++ b/arch/powerpc/kernel/vdso/Makefile
> >>>>> @@ -43,6 +43,7 @@ ccflags-y := -fno-common -fno-builtin -DBUILD_VDSO
> >>>>>   ccflags-y += $(DISABLE_LATENT_ENTROPY_PLUGIN)
> >>>>>   ccflags-y += $(call cc-option, -fno-stack-protector)
> >>>>>   ccflags-y += -DDISABLE_BRANCH_PROFILING
> >>>>> +ccflags-y += $(CONFIG_CC_OPT_INLINE_MEMSET)
> >>>>>   ccflags-y += -ffreestanding -fasynchronous-unwind-tables
> >>>>>   ccflags-remove-y := $(CC_FLAGS_FTRACE)
> >>>>>   ldflags-y := -Wl,--hash-style=both -nostdlib -shared -z noexecstack $(CLANG_FLAGS)
> >>>>> diff --git a/arch/riscv/kernel/vdso/Makefile b/arch/riscv/kernel/vdso/Makefile
> >>>>> index 8dbf2532a573..c023046a3fd7 100644
> >>>>> --- a/arch/riscv/kernel/vdso/Makefile
> >>>>> +++ b/arch/riscv/kernel/vdso/Makefile
> >>>>> @@ -36,6 +36,7 @@ endif
> >>>>>   ccflags-y := -fno-stack-protector
> >>>>>   ccflags-y += -DDISABLE_BRANCH_PROFILING
> >>>>>   ccflags-y += -fno-builtin
> >>>>> +ccflags-y += $(CONFIG_CC_OPT_INLINE_MEMSET)
> >>>>>   ccflags-y += $(KBUILD_BASE_ISA)$(CFI_MARCH)
> >>>>>   ccflags-y += $(CFI_FULL)
> >>>>>   asflags-y += $(KBUILD_BASE_ISA)$(CFI_MARCH)
> >>>>> diff --git a/arch/s390/kernel/vdso/Makefile b/arch/s390/kernel/vdso/Makefile
> >>>>> index 35c834b895ec..54bcf2984ca1 100644
> >>>>> --- a/arch/s390/kernel/vdso/Makefile
> >>>>> +++ b/arch/s390/kernel/vdso/Makefile
> >>>>> @@ -29,6 +29,7 @@ KBUILD_CFLAGS_VDSO := $(filter-out -munaligned-symbols,$(KBUILD_CFLAGS_VDSO))
> >>>>>   KBUILD_CFLAGS_VDSO := $(filter-out -fno-asynchronous-unwind-tables,$(KBUILD_CFLAGS_VDSO))
> >>>>>   KBUILD_CFLAGS_VDSO += -fPIC -fno-common -fno-builtin -fasynchronous-unwind-tables
> >>>>>   KBUILD_CFLAGS_VDSO += -fno-stack-protector $(DISABLE_KSTACK_ERASE)
> >>>>> +KBUILD_CFLAGS_VDSO += $(CONFIG_CC_OPT_INLINE_MEMSET)
> >>>>>   ldflags-y := -shared -soname=linux-vdso.so.1 \
> >>>>>               --hash-style=both --build-id=sha1 \
> >>>>>               $(call ld-option, --eh-frame-hdr) -T
> >>>>> diff --git a/arch/x86/entry/vdso/vdso64/Makefile b/arch/x86/entry/vdso/vdso64/Makefile
> >>>>> index 7c0790065b5e..c2353a065279 100644
> >>>>> --- a/arch/x86/entry/vdso/vdso64/Makefile
> >>>>> +++ b/arch/x86/entry/vdso/vdso64/Makefile
> >>>>> @@ -14,7 +14,7 @@ vobjs-$(CONFIG_X86_SGX)                       += vsgx.o
> >>>>>   vobjs-$(CONFIG_FUTEX_ROBUST_UNLOCK)    += vfutex.o
> >>>>>
> >>>>>   # Compilation flags
> >>>>> -flags-y                                := -DBUILD_VDSO64 -m64 -mcmodel=small
> >>>>> +flags-y                                := -DBUILD_VDSO64 -m64 -mcmodel=small $(CONFIG_CC_OPT_INLINE_MEMSET)
> >>>>>
> >>>>>   # The location of this include matters!
> >>>>>   include $(src)/../common/Makefile.include
> >>>>> diff --git a/init/Kconfig b/init/Kconfig
> >>>>> index 8583d9f06c52..ff3f8475dd2f 100644
> >>>>> --- a/init/Kconfig
> >>>>> +++ b/init/Kconfig
> >>>>> @@ -173,6 +173,13 @@ config CC_HAS_ALLOC_TOKEN
> >>>>>   config CC_HAS_MULTIDIMENSIONAL_NONSTRING
> >>>>>          def_bool $(success,echo 'char tag[][4] __attribute__((__nonstring__)) = { };' | $(CC) $(CLANG_FLAGS) -x c - -c -o /dev/null -Werror)
> >>>>>
> >>>>> +config CC_HAS_OPT_INLINE_MEMSET
> >>>>> +       def_bool $(cc-option,-finline-stringops=memset)
> >>>>> +
> >>>>> +config CC_OPT_INLINE_MEMSET
> >>>>> +       string
> >>>>> +       default "-finline-stringops=memset" if CC_HAS_OPT_INLINE_MEMSET
> >>>>> +
> >>>>>   config LD_CAN_USE_KEEP_IN_OVERLAY
> >>>>>          # ld.lld prior to 21.0.0 did not support KEEP within an overlay description
> >>>>>          # https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fllvm%2Fllvm-project%2Fpull%2F130661&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7C4df96063ffd94e6204df08df1bb54e3d%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C639260137659727352%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=03eAQvPTDs%2FN28bYiiZAVYcx2JFSkuOM7x2iwFU%2F%2FxA%3D&reserved=0
> >>>>> diff --git a/lib/vdso/getrandom.c b/lib/vdso/getrandom.c
> >>>>> index 2851afa9154f..f5cad5641985 100644
> >>>>> --- a/lib/vdso/getrandom.c
> >>>>> +++ b/lib/vdso/getrandom.c
> >>>>> @@ -29,6 +29,22 @@
> >>>>>          }                                                                       \
> >>>>>   } while (0)
> >>>>>
> >>>>> +#if __has_builtin(__builtin_memset_inline)
> >>>>> +#define memset_inline(dst, value, size) __builtin_memset_inline(dst, value, size)
> >>>>> +#elif IS_ENABLED(CONFIG_CC_HAS_OPT_INLINE_MEMSET)
> >>>>> +#define memset_inline(dst, value, size) __builtin_memset(dst, value, size)
> >>>>> +#else
> >>>>> +static inline void *memset_inline(void *dst, int value, size_t size)
> >>>>> +{
> >>>>> +       char *d = dst;
> >>>>> +
> >>>>> +       while (size--)
> >>>>> +               *d++ = value;
> >>>>> +
> >>>>> +       return d;
> >>>>> +}
> >>>>> +#endif
> >>>>
> >>>> Does this work?
> >>>> https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgodbolt.org%2Fz%2FTx6EaGMfb&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7C4df96063ffd94e6204df08df1bb54e3d%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C639260137659741160%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=Xv0rle85Js5BH2qNUL4q2Iv2ZwgJhJ4FaTuHYAvvEVo%3D&reserved=0
> >>>
> >>> Hmmm...possibly.
> >>> https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgodbolt.org%2Fz%2Fec9rzd9Yf&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7C4df96063ffd94e6204df08df1bb54e3d%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C639260137659754022%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=bLajY6c3CbYOZMJlBA4bYlVE3bNVc%2Fxe1KYk0rUJgjM%3D&reserved=0
> >>> I don't get it...
> >>
> >> Just keep it under 2 pages, it will be fine:
> >> https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgodbolt.org%2Fz%2FqqoMqfxev&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7C4df96063ffd94e6204df08df1bb54e3d%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C639260137659763804%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=Bcp1%2B%2Fir05tsPwf08OQkJx4vDkPliaSppam1e%2BTPprk%3D&reserved=0
> > 
> > https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgodbolt.org%2Fz%2F9W9f5a7Wx&data=05%7C02%7Cchristophe.leroy%40csgroup.eu%7C4df96063ffd94e6204df08df1bb54e3d%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C639260137659773169%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=YPGYXcKrnM4e1Q2nHdshRUaMT36w%2BRkHuIWIgcbpA94%3D&reserved=0
> > 
> > Apparently not on RISCV, which is what prompted this patch in the first
> > place. So I suspect Nathan's v2 here is the way to go.
> 
> 
> Don't you have -fno-builtin on RISCV like we have on powerpc ?

I think Nick mentioned this led to other problems and didn't exactly
solve the problem.