[PATCH -next v21 14/27] riscv: signal: Add sigcontext save/restore for vector

[Aurelien Jarno]

Hi,

On 2023-06-05 11:07, Andy Chiu wrote:
> From: Greentime Hu <greentime.hu at sifive.com>
> 
> This patch facilitates the existing fp-reserved words for placement of
> the first extension's context header on the user's sigframe. A context
> header consists of a distinct magic word and the size, including the
> header itself, of an extension on the stack. Then, the frame is followed
> by the context of that extension, and then a header + context body for
> another extension if exists. If there is no more extension to come, then
> the frame must be ended with a null context header. A special case is
> rv64gc, where the kernel support no extensions requiring to expose
> additional regfile to the user. In such case the kernel would place the
> null context header right after the first reserved word of
> __riscv_q_ext_state when saving sigframe. And the kernel would check if
> all reserved words are zeros when a signal handler returns.
> 
> __riscv_q_ext_state---->|	|<-__riscv_extra_ext_header
> 			~	~
> 	.reserved[0]--->|0	|<-	.reserved
> 		<-------|magic	|<-	.hdr
> 		|	|size	|_______ end of sc_fpregs
> 		|	|ext-bdy|
> 		|	~	~
> 	+)size	------->|magic	|<- another context header
> 			|size	|
> 			|ext-bdy|
> 			~	~
> 			|magic:0|<- null context header
> 			|size:0	|
> 
> The vector registers will be saved in datap pointer. The datap pointer
> will be allocated dynamically when the task needs in kernel space. On
> the other hand, datap pointer on the sigframe will be set right after
> the __riscv_v_ext_state data structure.

It appears that this patch somehow breaks userland, at least the rust
compiler. This can be observed for instance by building the rust-lsd
package in Debian, but many other rust packages are also affected:

* Failed build with kernel 6.5.3:
  https://buildd.debian.org/status/fetch.php?pkg=rust-lsd&arch=riscv64&ver=0.23.1-7%2Bb1&stamp=1696475386&raw=0

* Successful build with kernel 6.4.13:
  https://buildd.debian.org/status/fetch.php?pkg=rust-lsd&arch=riscv64&ver=0.23.1-7%2Bb1&stamp=1696491025&raw=0

It happens on hardware which does not have the V extension (in the above
case on a Hifive Unmatched board). This can also be reproduced in a QEMU
VM. Unfortunately disabling CONFIG_RISCV_ISA_V does not workaround the
issue.

It is not clear to me if it is a kernel issue or a wrong assumption on
the rust side. Any hint on how to continue investigating?

Regards
Aurelien

-- 
Aurelien Jarno                          GPG: 4096R/1DDD8C9B
aurelien at aurel32.net                     http://aurel32.net