Re: [PATCH v3] LoongArch: vDSO: Emit GNU_EH_FRAME correctly
From: Xi Ruoyao
Date: Tue Mar 03 2026 - 04:12:51 EST
On Tue, 2026-03-03 at 16:54 +0800, Huacai Chen wrote:
> Hi, Ruoyao,
>
> On Tue, Mar 3, 2026 at 4:33 PM Xi Ruoyao <xry111@xxxxxxxxxxx> wrote:
> >
> > With -fno-asynchronous-unwind-tables and --no-eh-frame-hdr (the default
> > of the linker), the GNU_EH_FRAME segment (specified by vdso.lds.S) is
> > empty. This is not valid, as the current DWARF specification mandates
> > the first byte of the EH frame to be the version number 1. It causes
> > some unwinders to complain, for example the ClickHouse query profiler
> > spams the log with messages:
> >
> > clickhouse-server[365854]: libunwind: unsupported .eh_frame_hdr
> > version: 127 at 7ffffffb0000
> >
> > Here "127" is just the byte located at the p_vaddr (0, i.e. the
> > beginning of the vDSO) of the empty GNU_EH_FRAME segment.
> > Cross-checking with /proc/365854/maps has also proven 7ffffffb0000 is
> > the start of vDSO in the process VM image.
> >
> > In LoongArch the -fno-asynchronous-unwind-tables option seems just a
> > MIPS legacy, and MIPS only uses this option to satisfy the MIPS-specific
> > "genvdso" program, per the commit cfd75c2db17e ("MIPS: VDSO: Explicitly
> > use -fno-asynchronous-unwind-tables"). IIRC it indicates some inherent
> > limitation of the MIPS ELF ABI and has nothing to do with LoongArch. So
> > we can simply flip it over to -fasynchronous-unwind-tables and pass
> > --eh-frame-hdr for linking the vDSO, allowing the profilers to unwind the
> > stack for statistics even if the sample point is taken when the PC is in
> > the vDSO.
> >
> > However simply adjusting the options above would exploit an issue: when
> > the libgcc unwinder saw the invalid GNU_EH_FRAME segment, it silently
> > falled back to a machine-specific routine to match the code pattern of
> > rt_sigreturn and extract the registers saved in the sigframe if the code
> > pattern is matched. As unwinding from signal handlers is vital for
> > libgcc to support pthread cancellation etc., the fall-back routine had
> > been silently keeping the LoongArch Linux systems functioning since
> > Linux 5.19. But when we start to emit GNU_EH_FRAME with the correct
> > format, fall-back routine will no longer be used and libgcc will fail
> > to unwind the sigframe, and unwinding from signal handlers will no
> > longer work, causing dozens of glibc test failures. To make it possible
> > to unwind from signal handlers again, it's necessary to code the unwind
> > info in __vdso_rt_sigreturn via .cfi_* directives.
> >
> > The offsets in the .cfi_* directives depend on the layout of struct
> > sigframe, notably the offset of sigcontect in the sigframe. To use the
> > offset in the assembly file, factor out struct sigframe into a header to
> > allow asm-offsets.c to output the offset for assembly.
> >
> > To work around a long-term issue in the libgcc unwinder (the pc is
> > unconditionally substracted by 1: doing so is technically incorrect for
> > a signal frame), a nop instruction is included with the two real
> > instructions in __vdso_rt_sigreturn in the same FDE PC range. The same
> > hack has been used on x86 for a long time.
> >
> > Fixes: c6b99bed6b8f ("LoongArch: Add VDSO and VSYSCALL support")
> > Cc: stable@xxxxxxxxxxxxxxx
> > Signed-off-by: Xi Ruoyao <xry111@xxxxxxxxxxx>
> > ---
> >
> > Changes from [v2]:
> > - Wrap .cfi_* for signal trampoline in SYM_SIGFUNC_START.
> > - Remove comment lines in sigframe.h not so meaningful.
> >
> > Changes from [v1] to v2:
> > - Use DWARF column 0 instead of the libgcc-specific column 72.
> > - Style change to sigframe.h.
> >
> > [v1]: https://lore.kernel.org/20260225104607.3803060-1-xry111@xxxxxxxxxxx
> >
> > arch/loongarch/include/asm/linkage.h | 34 +++++++++++++++++++++++++++
> > arch/loongarch/include/asm/sigframe.h | 9 +++++++
> > arch/loongarch/kernel/asm-offsets.c | 2 ++
> > arch/loongarch/kernel/signal.c | 6 +----
> > arch/loongarch/vdso/Makefile | 4 ++--
> > arch/loongarch/vdso/sigreturn.S | 10 +++-----
> > 6 files changed, 51 insertions(+), 14 deletions(-)
> > create mode 100644 arch/loongarch/include/asm/sigframe.h
> >
> > diff --git a/arch/loongarch/include/asm/linkage.h b/arch/loongarch/include/asm/linkage.h
> > index e2eca1a25b4e..db465036385f 100644
> > --- a/arch/loongarch/include/asm/linkage.h
> > +++ b/arch/loongarch/include/asm/linkage.h
> > @@ -42,3 +42,37 @@
> > SYM_END(name, SYM_T_NONE)
> >
> > #endif
> > +
> > +/*
> > + * This is for the signal handler trampoline, which is used as the return
> > + * address of the signal handlers in userspace instead of called normally.
> > + * The long standing libgcc bug https://gcc.gnu.org/PR124050 requires a
> > + * nop between .cfi_startproc and the actual address of the trampoline, so
> > + * we cannot simply use SYM_FUNC_START.
> > + *
> > + * This wrapper also contains all the .cfi_* directives for recovering
> > + * the content of the GPRs and the "return address" (where the rt_sigreturn
> > + * syscall will jump to), assuming there is a struct rt_sigframe (where
> > + * a struct sigcontext containing those information we need to recover) at
> > + * $sp. The "DWARF for the LoongArch(TM) Architecture" manual states
> > + * column 0 is for $zero, but it does not make too much sense to
> > + * save/restore the hardware zero register. Repurpose this column here
> > + * for the return address (here it's not the content of $ra we cannot use
> > + * the default column 3).
> > + */
> > +#define SYM_SIGFUNC_START(name) \
> > + .cfi_startproc; \
> > + .cfi_signal_frame; \
> > + .cfi_def_cfa 3, RT_SIGFRAME_SC; \
> > + .cfi_return_column 0; \
> > + .cfi_offset 0, SC_PC; \
> > + .irp num, 1, 2, 3, 4, 5, 6, 7, 8, \
> > + 9, 10, 11, 12, 13, 14, 15, 16, \
> > + 17, 18, 19, 20, 21, 22, 23, 24, \
> > + 25, 26, 27, 28, 29, 30, 31; \
> > + .cfi_offset \num, SC_REGS + \num * SZREG; \
> > + .endr; \
> > + nop; \
> > + SYM_START(name, SYM_L_GLOBAL, SYM_A_ALIGN)
> > +
> > +#define SYM_SIGFUNC_END(name) SYM_FUNC_END(name)
> Why this block is out of #endif ?
>
> > diff --git a/arch/loongarch/include/asm/sigframe.h b/arch/loongarch/include/asm/sigframe.h
> > new file mode 100644
> > index 000000000000..109298b8d7e0
> > --- /dev/null
> > +++ b/arch/loongarch/include/asm/sigframe.h
> > @@ -0,0 +1,9 @@
> > +/* SPDX-License-Identifier: GPL-2.0+ */
> > +
> > +#include <asm/siginfo.h>
> > +#include <asm/ucontext.h>
> > +
> > +struct rt_sigframe {
> > + struct siginfo rs_info;
> > + struct ucontext rs_uctx;
> > +};
> > diff --git a/arch/loongarch/kernel/asm-offsets.c b/arch/loongarch/kernel/asm-offsets.c
> > index 3017c7157600..2cc953f113ac 100644
> > --- a/arch/loongarch/kernel/asm-offsets.c
> > +++ b/arch/loongarch/kernel/asm-offsets.c
> > @@ -16,6 +16,7 @@
> > #include <asm/ptrace.h>
> > #include <asm/processor.h>
> > #include <asm/ftrace.h>
> > +#include <asm/sigframe.h>
> > #include <vdso/datapage.h>
> >
> > static void __used output_ptreg_defines(void)
> > @@ -220,6 +221,7 @@ static void __used output_sc_defines(void)
> > COMMENT("Linux sigcontext offsets.");
> > OFFSET(SC_REGS, sigcontext, sc_regs);
> > OFFSET(SC_PC, sigcontext, sc_pc);
> > + OFFSET(RT_SIGFRAME_SC, rt_sigframe, rs_uctx.uc_mcontext);
> > BLANK();
> > }
> >
> > diff --git a/arch/loongarch/kernel/signal.c b/arch/loongarch/kernel/signal.c
> > index c9f7ca778364..e297d54ea638 100644
> > --- a/arch/loongarch/kernel/signal.c
> > +++ b/arch/loongarch/kernel/signal.c
> > @@ -37,6 +37,7 @@
> > #include <asm/lbt.h>
> > #include <asm/ucontext.h>
> > #include <asm/vdso.h>
> > +#include <asm/sigframe.h>
> >
> > #ifdef DEBUG_SIG
> > # define DEBUGP(fmt, args...) printk("%s: " fmt, __func__, ##args)
> > @@ -51,11 +52,6 @@
> > #define lock_lbt_owner() ({ preempt_disable(); pagefault_disable(); })
> > #define unlock_lbt_owner() ({ pagefault_enable(); preempt_enable(); })
> >
> > -struct rt_sigframe {
> > - struct siginfo rs_info;
> > - struct ucontext rs_uctx;
> > -};
> > -
> > struct _ctx_layout {
> > struct sctx_info *addr;
> > unsigned int size;
> > diff --git a/arch/loongarch/vdso/Makefile b/arch/loongarch/vdso/Makefile
> > index 520f1513f07d..294c16b9517f 100644
> > --- a/arch/loongarch/vdso/Makefile
> > +++ b/arch/loongarch/vdso/Makefile
> > @@ -26,7 +26,7 @@ cflags-vdso := $(ccflags-vdso) \
> > $(filter -W%,$(filter-out -Wa$(comma)%,$(KBUILD_CFLAGS))) \
> > -std=gnu11 -fms-extensions -O2 -g -fno-strict-aliasing -fno-common -fno-builtin \
> > -fno-stack-protector -fno-jump-tables -DDISABLE_BRANCH_PROFILING \
> > - $(call cc-option, -fno-asynchronous-unwind-tables) \
> > + $(call cc-option, -fasynchronous-unwind-tables) \
> > $(call cc-option, -fno-stack-protector)
> > aflags-vdso := $(ccflags-vdso) \
> > -D__ASSEMBLY__ -Wa,-gdwarf-2
> > @@ -41,7 +41,7 @@ endif
> >
> > # VDSO linker flags.
> > ldflags-y := -Bsymbolic --no-undefined -soname=linux-vdso.so.1 \
> > - $(filter -E%,$(KBUILD_CFLAGS)) -shared --build-id -T
> > + $(filter -E%,$(KBUILD_CFLAGS)) -shared --build-id --eh-frame-hdr -T
> >
> > #
> > # Shared build commands.
> > diff --git a/arch/loongarch/vdso/sigreturn.S b/arch/loongarch/vdso/sigreturn.S
> > index 9cb3c58fad03..e40bf4186f29 100644
> > --- a/arch/loongarch/vdso/sigreturn.S
> > +++ b/arch/loongarch/vdso/sigreturn.S
> > @@ -12,13 +12,9 @@
> >
> > #include <asm/regdef.h>
> > #include <asm/asm.h>
> > +#include <asm/asm-offsets.h>
> >
> > - .section .text
> Can we keep this line?
Oops, indeed this should be kept. I'm unsure why I didn't see any issue
without it...
--
Xi Ruoyao <xry111@xxxxxxxxxxx>