From: Huacai Chen <chenhuacai@kernel.org>
To: Xi Ruoyao <xry111@xry111.site>
Cc: WANG Xuerui <kernel@xen0n.name>,
Jinyang He <hejinyang@loongson.cn>,
WANG Rui <wangrui@loongson.cn>, Mingcong Bai <jeffbai@aosc.io>,
Zixing Liu <liushuyu@aosc.io>, "H . Peter Anvin" <hpa@zytor.com>,
stable@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>,
Menglong Dong <menglong8.dong@gmail.com>,
Bibo Mao <maobibo@loongson.cn>,
Tiezhu Yang <yangtiezhu@loongson.cn>,
Hanlu Li <lihanlu@loongson.cn>,
Nathan Chancellor <nathan@kernel.org>,
Jiaxun Yang <jiaxun.yang@flygoat.com>,
Ard Biesheuvel <ardb@kernel.org>,
Wentao Guan <guanwentao@uniontech.com>,
loongarch@lists.linux.dev, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v4] LoongArch: vDSO: Emit GNU_EH_FRAME correctly
Date: Thu, 26 Mar 2026 15:01:45 +0800 [thread overview]
Message-ID: <CAAhV-H6RPEJ+K0bObgSR6K7-DSxiEzVpOpevBYuiAXK3H4wGzA@mail.gmail.com> (raw)
In-Reply-To: <20260312101440.772081-1-xry111@xry111.site>
Applied, thanks.
Huacai
On Thu, Mar 12, 2026 at 6:15 PM Xi Ruoyao <xry111@xry111.site> 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@vger.kernel.org
> Signed-off-by: Xi Ruoyao <xry111@xry111.site>
> ---
>
> Changes from [v3]:
> - Move SYM_SIGFUNC_* into include guard.
> - Retain ".section .text".
>
> Changes from [v2] to v3:
> - 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.
>
> [v3]: https://lore.kernel.org/20260303083248.567185-1-xry111@xry111.site
> [v2]: https://lore.kernel.org/20260227072031.581229-1-xry111@xry111.site
> [v1]: https://lore.kernel.org/20260225104607.3803060-1-xry111@xry111.site
>
> 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 | 8 +++----
> 6 files changed, 51 insertions(+), 12 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..9f10de85b3f6 100644
> --- a/arch/loongarch/include/asm/linkage.h
> +++ b/arch/loongarch/include/asm/linkage.h
> @@ -41,4 +41,38 @@
> .cfi_endproc; \
> SYM_END(name, SYM_T_NONE)
>
> +/*
> + * 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)
> +
> #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..a876dbdc458a 100644
> --- a/arch/loongarch/vdso/sigreturn.S
> +++ b/arch/loongarch/vdso/sigreturn.S
> @@ -12,13 +12,11 @@
>
> #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)
>
> +SYM_SIGFUNC_START(__vdso_rt_sigreturn)
> li.w a7, __NR_rt_sigreturn
> syscall 0
> -
> -SYM_FUNC_END(__vdso_rt_sigreturn)
> +SYM_SIGFUNC_END(__vdso_rt_sigreturn)
> --
> 2.53.0
>
prev parent reply other threads:[~2026-03-26 7:01 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-03-12 10:14 [PATCH v4] LoongArch: vDSO: Emit GNU_EH_FRAME correctly Xi Ruoyao
2026-03-26 7:01 ` Huacai Chen [this message]
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