linux-kernel - Re: [PATCH v4] scripts/gdb: add lx

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Date:   Tue, 15 Nov 2022 21:23:29 +0000
From:   <Conor.Dooley@...rochip.com>
To:     <debug@...osinc.com>
CC:     <ajones@...tanamicro.com>, <aou@...s.berkeley.edu>,
        <jan.kiszka@...mens.com>, <kbingham@...nel.org>,
        <linux-kernel@...r.kernel.org>, <linux-riscv@...ts.infradead.org>,
        <palmer@...belt.com>, <paul.walmsley@...ive.com>
Subject: Re: [PATCH v4] scripts/gdb: add lx_current support for riscv

Hey Deepak,

On 15/11/2022 20:40, Deepak Gupta wrote:
> EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
> 
> csr_sscratch CSR holds current task_struct address when hart is in
> user space. Trap handler on entry spills csr_sscratch into "tp" (x2)
> register and zeroes out csr_sscratch CSR. Trap handler on exit reloads
> "tp" with expected user mode value and place current task_struct address
> again in csr_sscratch CSR.
> 
> This patch assumes "tp" is pointing to task_struct. If value in
> csr_sscratch is numerically greater than "tp" then it assumes csr_sscratch
> is correct address of current task_struct. This logic holds when
>    - hart is in user space, "tp" will be less than csr_sscratch.
>    - hart is in kernel space but not in trap handler, "tp" will be more
>      than csr_sscratch (csr_sscratch being equal to 0).
>    - hart is executing trap handler
>        - "tp" is still pointing to user mode but csr_sscratch contains
>           ptr to task_struct. Thus numerically higher.
>        - "tp" is  pointing to task_struct but csr_sscratch now contains
>           either 0 or numerically smaller value (transiently holds
>           user mode tp)
> 
> Patch also adds new cached type "ulong" in scripts/gdb/linux/utils.py
> 

> Since patch has changed a little bit from v1 and I didn't include
> changelog earlier, here it is.

This bit here needs to go below the --- line as it should not end up
in the commit logs (as everything below the --- is ignored.

> 
> ---
> v1 --> v2:
>  - added logic to locate task_struct irrespective of priv
>  - made locating task_struct agnostic to bitness(32 vs 64).
>  - added caching of ulong type in scripts/gdb/linux/utils.py
>  - added more descriptive commit message
> 
> v2 --> v3:
>  - amended commit message and source line to fit column width
> 
> v3 --> v4:
>  - amended commit message and remove whitespace in source
>  - added Reviewed-by for reviewers
> ---
> 
> Signed-off-by: Deepak Gupta <debug@...osinc.com>
> Reviewed-by: Andrew Jones <ajones@...tanamicro.com>

And this bit needs to go above the first --- line as it does need
to go into the commit logs.

The order is:
$subject

body

tags
---
changelog & comments

diffs

For example, this is what I see when I apply the patch:

$ b4 shazam 20221115204003.1866421-1-debug@...osinc.com
Grabbing thread from lore.kernel.org/all/20221115204003.1866421-1-debug%40rivosinc.com/t.mbox.gz
Checking for newer revisions on https://lore.kernel.org/all/
Analyzing 13 messages in the thread
Will use the latest revision: v4
You can pick other revisions using the -vN flag
Checking attestation on all messages, may take a moment...
---
  ✓ [PATCH v4] scripts/gdb: add lx_current support for riscv
  ---
  ✓ Signed: DKIM/rivosinc-com.20210112.gappssmtp.com (From: debug@...osinc.com)
---
Total patches: 1
---
Applying: scripts/gdb: add lx_current support for riscv

$ git show
commit 5c93617c68980f767e312fc51849d78093f56e72 (HEAD)
Author: Deepak Gupta <debug@...osinc.com>
Date:   Tue Nov 15 12:40:03 2022 -0800

    scripts/gdb: add lx_current support for riscv
    
    csr_sscratch CSR holds current task_struct address when hart is in
    user space. Trap handler on entry spills csr_sscratch into "tp" (x2)
    register and zeroes out csr_sscratch CSR. Trap handler on exit reloads
    "tp" with expected user mode value and place current task_struct address
    again in csr_sscratch CSR.
    
    This patch assumes "tp" is pointing to task_struct. If value in
    csr_sscratch is numerically greater than "tp" then it assumes csr_sscratch
    is correct address of current task_struct. This logic holds when
       - hart is in user space, "tp" will be less than csr_sscratch.
       - hart is in kernel space but not in trap handler, "tp" will be more
         than csr_sscratch (csr_sscratch being equal to 0).
       - hart is executing trap handler
           - "tp" is still pointing to user mode but csr_sscratch contains
              ptr to task_struct. Thus numerically higher.
           - "tp" is  pointing to task_struct but csr_sscratch now contains
              either 0 or numerically smaller value (transiently holds
              user mode tp)
    
    Patch also adds new cached type "ulong" in scripts/gdb/linux/utils.py
    
    Since patch has changed a little bit from v1 and I didn't include
    changelog earlier, here it is.

Taken a wee bit of the back and forth, Looks like you're nearly
there though!

Conor.