Re: [PATCH] riscv: eliminate unreliable __builtin_frame_address(1)

[Andreas Schwab <schwab@linux-m68k.org>]

On Jan 17 2022, Changbin Du wrote:

> I tried different pieces of code which uses __builtin_frame_address(1)
> (with both gcc version 7.5.0 and 10.3.0) to verify whether it works as
> expected on riscv64. The result is negative.
>
> What the compiler had generated is as below:
> 31                      fp = (unsigned long)__builtin_frame_address(1);
>    0xffffffff80006024 <+200>:   ld      s1,0(s0)
>
> It takes '0(s0)' as the address of frame 1 (caller), but the actual address
> should be '-16(s0)'.
>
>           |       ...       | <-+
>           +-----------------+   |
>           | return address  |   |
>           | previous fp     |   |
>           | saved registers |   |
>           | local variables |   |
>   $fp --> |       ...       |   |
>           +-----------------+   |
>           | return address  |   |
>           | previous fp --------+
>           | saved registers |
>   $sp --> | local variables |
>           +-----------------+
>
> This leads the kernel can not dump the full stack trace on riscv.
>
> [    7.222126][    T1] Call Trace:
> [    7.222804][    T1] [<ffffffff80006058>] dump_backtrace+0x2c/0x3a
>
> This problem is not exposed on most riscv builds just because the '0(s0)'
> occasionally is the address frame 2 (caller's caller), if only ra and fp
> are stored in frame 1 (caller).
>
>           |       ...       | <-+
>           +-----------------+   |
>           | return address  |   |
>   $fp --> | previous fp     |   |
>           +-----------------+   |
>           | return address  |   |
>           | previous fp --------+
>           | saved registers |
>   $sp --> | local variables |
>           +-----------------+
>
> This could be a *bug* of gcc that should be fixed.

Yes, it would be nice to get this fixed.  The riscv target does not
override DYNAMIC_CHAIN_ADDRESS, thus the default is used, which has the
noted effect.

-- 
Andreas Schwab, schwab@linux-m68k.org
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