Re: [PATCH v4] scripts/gdb: add lx_current support for riscv

Conor.Dooley
2022-11-15T21:24:55Z

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@rivosinc.com>
> Reviewed-by: Andrew Jones <ajones@ventanamicro.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@rivosinc.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@rivosinc.com)
---
Total patches: 1
---
Applying: scripts/gdb: add lx_current support for riscv

$ git show
commit 5c93617c68980f767e312fc51849d78093f56e72 (HEAD)
Author: Deepak Gupta <debug@rivosinc.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.