Re: [PATCH for 6.6] LoongArch: vDSO: Emit GNU_EH_FRAME correctly

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Mon, Mar 30, 2026 at 06:01:33PM +0800, Huacai Chen wrote:
> From: Xi Ruoyao <xry111@xry111.site>
> 
> commit e4878c37f6679fdea91b27a0f4e60a871f0b7bad upstream.
> 
> 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 sigcontext 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.
> 
> Cc: stable@vger.kernel.org
> Fixes: c6b99bed6b8f ("LoongArch: Add VDSO and VSYSCALL support")
> Signed-off-by: Xi Ruoyao <xry111@xry111.site>
> Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
> ---

Does not apply cleanly on the latest 6.12.y queue :(