[PATCH v2] scripts/gdb: add lx_current support for riscv

Deepak Gupta
2022-11-15T01:29:37Z

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_scratch CSR.

This patch assumes "tp" is pointing to task_struct. If value in csr_scratch is numerically
greater than "tp" then it assumes csr_scratch is correct address of current task_struct.
This logic holds when
- hart is in user space, "tp" will be less than csr_scratch.
- hart is in kernel space but not in trap handler, "tp" will be more than csr_scratch.
- hart is executing trap handler
- "tp" is still pointing to user mode but csr_scratch contains ptr to task_struct.
numerically higher.
- "tp" is now pointing to task_struct but csr_scratch now contains either 0 or numerically
smaller value.

Patch also adds new cached type "ulong" in scripts/gdb/linux/utils.py

Signed-off-by: Deepak Gupta <debug@rivosinc.com>
---
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..fd818d7896ce 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.
+ # So when scratch register holds higher value (negative address as ulong is bigger 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)
+
+ 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)
--
2.25.1