linux-kernel - Re: [PATCH RFC] arch/riscv: add proc status (proc/<pid>/status)

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Message-ID: <aPl48xlYTohtI648@debug.ba.rivosinc.com>
Date: Wed, 22 Oct 2025 17:38:11 -0700
From: Deepak Gupta <debug@...osinc.com>
To: Heinrich Schuchardt <heinrich.schuchardt@...onical.com>
Cc: linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
	kito.cheng@...ive.com, charlie@...osinc.com, atishp@...osinc.com,
	alexghiti@...osinc.com, cleger@...osinc.com, zong.li@...ive.com,
	valentin.haudiquet@...onical.com, jesse.huang@...ive.com,
	Paul Walmsley <pjw@...nel.org>, Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>, Alexandre Ghiti <alex@...ti.fr>
Subject: Re: [PATCH RFC] arch/riscv: add proc status (proc/<pid>/status)

Hi Hienrich,


On Tue, Oct 21, 2025 at 01:13:36PM +0200, Heinrich Schuchardt wrote:
>On 10/20/25 20:18, Deepak Gupta wrote:
>>x86 has proc/<pid>/status to see various runtime characteristics
>>of running task. Implement same for riscv. This patch implements
>>status for shadow stack and landing pad state.
>>
>>Signed-off-by: Deepak Gupta <debug@...osinc.com>
>>---
>>This is just a one patch but sending it as RFC with cover letter to seek
>>feedback. x86 has `arch_proc_pid_thread_features` to enumerate status of
>>arch thread specific features on runtime. This can be done using prctl
>>as well but from quick/script perspective `cat /proc/<pid>/status` is more
>>desirable. In this patch, simply `arch_proc_pid_thread_features` is implemented
>>for riscv which queries shadow stack and landing pad state and reports back.
>>Thus it is dependent on riscv user cfi enabling series.
>
>Hello Deepak,
>
>This looks like a valuable addition.
>
>Shouldn't architecture specific fields be in
>/proc/<pid>/arch_status and not in /proc/<pid>/status?

Just following what x86 did. I believe shadow stack and landing pad are there
in arm64, x86 and riscv. So probably /proc/<pid>/status is best place.
>
>Please, add a documentation patch adding the RISC-V specific fields to
>"3.12  /proc/<pid>/arch_status - Task architecture specific information"
>in Documentation/filesystems/proc.rst.
>
>Should you stick to /proc/*/status, please, add the documentation to 
>Documentation/arch/riscv/.

Will do.

>
>>
>>If this patch itself is self-standing and good enough, I can roll this patch
>>as part of riscv user cfi enabling series.
>>
>>OR
>>
>>If there is ask for other riscv thread specific features that could be
>>enumerated in similar fashion, we will need it to be done as separate series
>>(with `thread_features` added to `thread_info`)
>>
>>Example output below.
>>
>>Name:   cat
>>Umask:  0022
>>State:  R (running)
>>Tgid:   133
>>Ngid:   0
>>Pid:    133
>>PPid:   129
>>TracerPid:      0
>>Uid:    0       0       0       0
>>Gid:    0       0       0       0
>>FDSize: 256
>>Groups: 0 10
>>NStgid: 133
>>NSpid:  133
>>NSpgid: 133
>>NSsid:  129
>>Kthread:        0
>>VmPeak:    10788 kB
>>VmSize:    10788 kB
>>VmLck:         0 kB
>>VmPin:         0 kB
>>VmHWM:      1400 kB
>>VmRSS:      1400 kB
>>RssAnon:             116 kB
>>RssFile:            1284 kB
>>RssShmem:              0 kB
>>VmData:       92 kB
>>VmStk:      8324 kB
>>VmExe:         4 kB
>>VmLib:      2312 kB
>>VmPTE:        40 kB
>>VmSwap:        0 kB
>>HugetlbPages:          0 kB
>>CoreDumping:    0
>>THP_enabled:    0
>>untag_mask:     0xffffffffffffffff
>>Threads:        1
>>SigQ:   0/31771
>>SigPnd: 0000000000000000
>>ShdPnd: 0000000000000000
>>SigBlk: 0000000000000000
>>SigIgn: 0000000000000000
>>SigCgt: 0000000000000000
>>CapInh: 0000000000000000
>>CapPrm: 000001ffffffffff
>>CapEff: 000001ffffffffff
>>CapBnd: 000001ffffffffff
>>CapAmb: 0000000000000000
>>NoNewPrivs:     0
>>Seccomp:        0
>>Seccomp_filters:        0
>>Speculation_Store_Bypass:       unknown
>>SpeculationIndirectBranch:      unsupported
>>Cpus_allowed:   3
>>Cpus_allowed_list:      0-1
>>Mems_allowed:   1
>>Mems_allowed_list:      0
>>voluntary_ctxt_switches:        0
>>nonvoluntary_ctxt_switches:     3
>>riscv_thread_features:  shstk_enabled lpad_enabled
>>riscv_thread_features_locked:   shstk_unlocked lpad_unlocked
>>---
>>  arch/riscv/kernel/Makefile     |  1 +
>>  arch/riscv/kernel/cpu/Makefile |  1 +
>>  arch/riscv/kernel/cpu/proc.c   | 28 ++++++++++++++++++++++++++++
>>  3 files changed, 30 insertions(+)
>>
>>diff --git a/arch/riscv/kernel/Makefile b/arch/riscv/kernel/Makefile
>>index f60fce69b725..b32c11667d81 100644
>>--- a/arch/riscv/kernel/Makefile
>>+++ b/arch/riscv/kernel/Makefile
>>@@ -71,6 +71,7 @@ obj-y	+= vendor_extensions.o
>>  obj-y	+= vendor_extensions/
>>  obj-y	+= probes/
>>  obj-y	+= tests/
>>+obj-y	+= cpu/
>>  obj-$(CONFIG_MMU) += vdso.o vdso/
>>  obj-$(CONFIG_RISCV_MISALIGNED)	+= traps_misaligned.o
>>diff --git a/arch/riscv/kernel/cpu/Makefile b/arch/riscv/kernel/cpu/Makefile
>>new file mode 100644
>>index 000000000000..2b474fb49afe
>>--- /dev/null
>>+++ b/arch/riscv/kernel/cpu/Makefile
>>@@ -0,0 +1 @@
>>+obj-$(CONFIG_PROC_FS)			+= proc.o
>>diff --git a/arch/riscv/kernel/cpu/proc.c b/arch/riscv/kernel/cpu/proc.c
>>new file mode 100644
>>index 000000000000..4661190c43d1
>>--- /dev/null
>>+++ b/arch/riscv/kernel/cpu/proc.c
>>@@ -0,0 +1,28 @@
>>+// SPDX-License-Identifier: GPL-2.0
>>+#include <linux/smp.h>
>>+#include <linux/timex.h>
>>+#include <linux/string.h>
>>+#include <linux/seq_file.h>
>>+#include <linux/cpufreq.h>
>>+#include <linux/proc_fs.h>
>>+#include <asm/usercfi.h>
>>+
>>+#ifdef CONFIG_RISCV_USER_CFI
>>+
>>+void arch_proc_pid_thread_features(struct seq_file *m, struct task_struct *task)
>>+{
>>+	seq_puts(m, "riscv_thread_features:\t");
>>+	if (is_shstk_enabled(task))
>>+		seq_puts(m, "shstk_enabled ");
>
>According to Documentation/arch/x86/shstk.rst, x86 is avoiding the 
>'_enabled' postfix here:
>
>x86_Thread_features: shstk wrss

I can follow same and use `shstk`. We don't have `wrss` because
shadow stack is writeable by right instruction (sspush and ssamoswap) in
riscv.

>
>>+
>>+	if (is_indir_lp_enabled(task))
>>+		seq_puts(m, "lpad_enabled ");

Would you prefer similar here. Just "lpad"?

>>+
>>+	seq_putc(m, '\n');
>>+
>>+	seq_puts(m, "riscv_thread_features_locked:\t");
>>+	is_shstk_locked(task) ? seq_puts(m, "shstk_locked ") : seq_puts(m, "shstk_unlocked ");
>>+	is_indir_lp_locked(task) ? seq_puts(m, "lpad_locked ") : seq_puts(m, "lpad_unlocked ");
>
>Why do we need any entry for an unlocked feature in 
>riscv_thread_features_locked?

absence of "feature_locked" is considered unlocked, then I can drop it.

>
>Best regards
>
>Heinrich
>
>>+	seq_putc(m, '\n');
>>+}
>>+#endif /* CONFIG_RISCV_USER_CFI */
>>
>>---
>>base-commit: 3a8660878839faadb4f1a6dd72c3179c1df56787
>>change-id: 20251017-proc_status-df1aedc85c7c
>>--
>>- debug
>>
>