Re: [PATCH v5] scripts/gdb: add lx_current support for riscv

From: Jan Kiszka
Date: Mon Jan 02 2023 - 04:09:11 EST


On 15.11.22 23:10, Deepak Gupta wrote:
> 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
>
> Signed-off-by: Deepak Gupta <debug@xxxxxxxxxxxx>
> Reviewed-by: Andrew Jones <ajones@xxxxxxxxxxxxxxxx>
>
> ---
> Since patch has changed a little bit from v1 and I didn't include
> changelog earlier, here it is.
>
> 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
>
> v4 --> v5:
> - changing the order of changelog and sign off/review tags in commit
> ---
> ---
> scripts/gdb/linux/cpus.py | 15 +++++++++++++++
> scripts/gdb/linux/utils.py | 5 +++++
> 2 files changed, 20 insertions(+)
>
> diff --git a/scripts/gdb/linux/cpus.py b/scripts/gdb/linux/cpus.py
> index 15fc4626d236..14c22f82449b 100644
> --- a/scripts/gdb/linux/cpus.py
> +++ b/scripts/gdb/linux/cpus.py
> @@ -173,6 +173,21 @@ def get_current_task(cpu):
> else:
> raise gdb.GdbError("Sorry, obtaining the current task is not allowed "
> "while running in userspace(EL0)")
> + elif utils.is_target_arch("riscv"):
> + current_tp = gdb.parse_and_eval("$tp")
> + scratch_reg = gdb.parse_and_eval("$sscratch")
> +
> + # by default tp points to current task
> + current_task = current_tp.cast(task_ptr_type)
> +
> + # scratch register is set 0 in trap handler after entering kernel.
> + # When hart is in user mode, scratch register is pointing to task_struct.
> + # and tp is used by user mode. So when scratch register holds larger value
> + # (negative address as ulong is larger value) than tp, then use scratch register.
> + if (scratch_reg.cast(utils.get_ulong_type()) > current_tp.cast(utils.get_ulong_type())):
> + current_task = scratch_reg.cast(task_ptr_type)

Why not if-else for the assignment here?

> +
> + return current_task.dereference()
> else:
> raise gdb.GdbError("Sorry, obtaining the current task is not yet "
> "supported with this arch")
> diff --git a/scripts/gdb/linux/utils.py b/scripts/gdb/linux/utils.py
> index 1553f68716cc..ddaf3089170d 100644
> --- a/scripts/gdb/linux/utils.py
> +++ b/scripts/gdb/linux/utils.py
> @@ -35,12 +35,17 @@ class CachedType:
>
>
> long_type = CachedType("long")
> +ulong_type = CachedType("ulong")
> atomic_long_type = CachedType("atomic_long_t")
>
> def get_long_type():
> global long_type
> return long_type.get_type()
>
> +def get_ulong_type():
> + global ulong_type
> + return ulong_type.get_type()
> +
> def offset_of(typeobj, field):
> element = gdb.Value(0).cast(typeobj)
> return int(str(element[field].address).split()[0], 16)

Looks good to me otherwise.

Jan

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