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

Deepak Gupta
2022-11-15T22:11:38Z

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@rivosinc.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>

---
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)
+
+ 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