Re: [PATCH] LoongArch: vDSO: Emit GNU_EH_FRAME correctly

[Huacai Chen]

Hi, Ruoyao,

On Wed, Feb 25, 2026 at 6:46 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").  IIUC 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>
> ---
>  arch/loongarch/include/asm/sigframe.h | 22 ++++++++++++++
>  arch/loongarch/kernel/asm-offsets.c   |  2 ++
>  arch/loongarch/kernel/signal.c        |  6 +---
>  arch/loongarch/vdso/Makefile          |  4 +--
>  arch/loongarch/vdso/sigreturn.S       | 44 ++++++++++++++++++++++++---
>  5 files changed, 67 insertions(+), 11 deletions(-)
>  create mode 100644 arch/loongarch/include/asm/sigframe.h
>
> diff --git a/arch/loongarch/include/asm/sigframe.h b/arch/loongarch/include/asm/sigframe.h
> new file mode 100644
> index 000000000000..6889bcf5dc88
> --- /dev/null
> +++ b/arch/loongarch/include/asm/sigframe.h
> @@ -0,0 +1,22 @@
> +/* SPDX-License-Identifier: GPL-2.0+ */
> +/*
> + * Separated from arch/loongarch/kernel/signal.c:
> + *
> + * Author: Hanlu Li <lihanlu@xxxxxxxxxxx>
> + *         Huacai Chen <chenhuacai@xxxxxxxxxxx>
> + * Copyright (C) 2020-2022 Loongson Technology Corporation Limited
> + *
> + * Derived from MIPS:
> + * Copyright (C) 1991, 1992  Linus Torvalds
> + * Copyright (C) 1994 - 2000  Ralf Baechle
> + * Copyright (C) 1999, 2000 Silicon Graphics, Inc.
> + * Copyright (C) 2014, Imagination Technologies Ltd.
> + */
I think we don't need to copy so many lines here, they are enough in signal.c.
> +
> +#include <uapi/asm/ucontext.h>
Is it a requirement that the UAPI header should be the first?

> +#include <asm/siginfo.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..e46c1deacb9e 100644
> --- a/arch/loongarch/vdso/sigreturn.S
> +++ b/arch/loongarch/vdso/sigreturn.S
> @@ -12,13 +12,49 @@
>
>  #include <asm/regdef.h>
>  #include <asm/asm.h>
> +#include <asm/asm-offsets.h>
>
>         .section        .text
> -       .cfi_sections   .debug_frame
>
> -SYM_FUNC_START(__vdso_rt_sigreturn)
> +       .cfi_startproc
> +       .cfi_signal_frame
>
> +       /*
> +        * There is a struct rt_sigframe at $sp, set CFA to the address of
> +        * the struct sigcontext in the rt_sigframe to simplify the
> +        * offsets below.
> +        */
> +       .cfi_def_cfa 3, RT_SIGFRAME_SC
> +
> +       /*
> +        * 72 is DWARF 2 CFA column for the return address from a signal
> +        * handler context on LoongArch, i.e. the PC stored in the
> +        * sigcontext.
> +        */
> +       .cfi_return_column 72
> +       .cfi_offset 72, SC_PC
> +
> +       /* The GPRs of the "caller" are also stored in the sigcontext.  */
> +.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
> +
> +       /*
> +        * HACK: The dwarf2 unwind routine will subtract 1 from the return
> +        * address to get an address in the middle of the persumed call
> +        * instruction.  While in libgcc there exists a logic to avoid
> +        * subtracting 1 for the signal frame (a frame with the 'S'
> +        * augmentation that we've already added via .cfi_signal_frame),
> +        * unfortunately it doesn't really work: the check of signal frame
> +        * is at libgcc/unwind-dw2:1008 in GCC 15.2.0, but the flag it
> +        * checks will only get updated by the extract_cie_info call at line
> +        * 1025.  So include a nop before the real start to make up for it.
> +        * This is also the reason we don't use SYM_FUNC_START.
> +        */
> +       nop
This hack is out of my knowledge, can "nop" be after SYM_START()?

Huacai

> +SYM_START(__vdso_rt_sigreturn, SYM_L_GLOBAL, SYM_A_ALIGN)
>         li.w    a7, __NR_rt_sigreturn
>         syscall 0
> -
> -SYM_FUNC_END(__vdso_rt_sigreturn)
> +       .cfi_endproc
> +SYM_END(__vdso_rt_sigreturn, SYM_T_FUNC)
> --
> 2.53.0
>
>