[PATCH] fix missing .debug_frame DWARF section if gcc-based toolchain is used

[Anup Patel]

On Mon, Apr 21, 2025 at 6:17 PM Parshintsev Anatoly
<anatoly.parshintsev at syntacore.com> wrote:
>
> When OpenSBI is built with a relatively new compiler (gcc-13 and greater)
> I observed that GDB is unable to produce proper backtraces and some
> variable values appear corrupted (even if the associated DWARF
> location descriptor is correct).
>
> Turns out that to properly work with debug information, debuggers often
> need to unwind the stack. They generally rely on Call Frame Information
> (CFI) records provided by the compiler to facilitate this task.
> Currently, the GCC compiler offers two mechanisms:
>
> - `.debug_frame` section (as described in the DWARF specification).
> - `.eh_frame` sections (as described in LSB documents).
>
> The latter (`.eh_frame`) supports stack unwinding at runtime, providing
> a framework for C++ exceptions or enabling backtrace generation using
> libraries like libunwind. However, the downside of this approach is that
> these sections should be part of loadable segments.
>
> The former (`.debug_frame`) is simply an ordinary debug section.
>
> Starting from GCC 13, Linux targets enable the `-fasynchronous-unwind-tables`
> and `-funwind-tables` flags by default. Relevant commit:
> https://github.com/gcc-mirror/gcc/commit/3cd08f7168
>
> When these flags are active, the compiler generates `.eh_frame` sections
> instead of `.debug_frame`. Since OpenSBI is built using the **Linux
> toolchain**, this behavior applies to OpenSBI as well.
>
> The problem arises because the SBI build system uses `-Wl,--gc-sections`, which
> discards the `.eh_frame` section.
>
> Possible Fixes:
>
> 1. Enforce `.debug_frame` generation – Modify compiler flags to generate
> `.debug_frame` instead of `.eh_frame`.
> 2. Preserve `.eh_frame` in the linker script – Add `KEEP(*(.eh_frame))` to
> ensure the section is not discarded.
>
> I chose Option 1 because it avoids any runtime overhead.
>
> Signed-off-by: Parshintsev Anatoly <anatoly.parshintsev at syntacore.com>

LGTM.

Reviewed-by: Anup Patel <anup at brainfault.org>

I have simplified the PATCH subject at the time of merging.

Applied this patch to the riscv/opensbi repo.

Thanks,
Anup

> ---
>  Makefile | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/Makefile b/Makefile
> index e90836c..85748bc 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -362,6 +362,7 @@ GENFLAGS    +=      $(firmware-genflags-y)
>
>  CFLAGS         =       -g -Wall -Werror -ffreestanding -nostdlib -fno-stack-protector -fno-strict-aliasing -ffunction-sections -fdata-sections
>  CFLAGS         +=      -fno-omit-frame-pointer -fno-optimize-sibling-calls
> +CFLAGS         +=      -fno-asynchronous-unwind-tables -fno-unwind-tables
>  # Optionally supported flags
>  ifeq ($(CC_SUPPORT_VECTOR),y)
>  CFLAGS         +=      -DOPENSBI_CC_SUPPORT_VECTOR
> --
> 2.43.0
>
>
>
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