[PATCH] rv: Fix wrong type cast in enabled_monitors_next()

[PATCH] rv: Fix wrong type cast in enabled_monitors_next()

[Nam Cao]

Argument 'p' of enabled_monitors_next() is not a pointer to struct
rv_monitor, it is actually a pointer to the list_head inside struct
rv_monitor. Therefore it is wrong to cast 'p' to struct rv_monitor *.

This wrong type cast has been there since the beginning. But it still
worked because the list_head was the first field in struct rv_monitor_def.
This is no longer true since commit 24cbfe18d55a ("rv: Merge struct
rv_monitor_def into struct rv_monitor") moved the list_head, and this wrong
type cast became a functional problem.

Properly use container_of() instead.

Fixes: 24cbfe18d55a ("rv: Merge struct rv_monitor_def into struct rv_monitor")
Signed-off-by: Nam Cao <namcao@linutronix.de>
---
 kernel/trace/rv/rv.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/trace/rv/rv.c b/kernel/trace/rv/rv.c
index bd7d56dbf6c2..6ce3495164d8 100644
--- a/kernel/trace/rv/rv.c
+++ b/kernel/trace/rv/rv.c
@@ -495,7 +495,7 @@ static void *available_monitors_next(struct seq_file *m, void *p, loff_t *pos)
  */
 static void *enabled_monitors_next(struct seq_file *m, void *p, loff_t *pos)
 {
-	struct rv_monitor *mon = p;
+	struct rv_monitor *mon = container_of(p, struct rv_monitor, list);
 
 	(*pos)++;
 
-- 
2.39.5

Re: [PATCH] rv: Fix wrong type cast in enabled_monitors_next()

[Gabriele Monaco]

On Wed, 2025-08-06 at 14:09 +0200, Nam Cao wrote:
> Argument 'p' of enabled_monitors_next() is not a pointer to struct
> rv_monitor, it is actually a pointer to the list_head inside struct
> rv_monitor. Therefore it is wrong to cast 'p' to struct rv_monitor *.
> 
> This wrong type cast has been there since the beginning. But it still
> worked because the list_head was the first field in struct
> rv_monitor_def.
> This is no longer true since commit 24cbfe18d55a ("rv: Merge struct
> rv_monitor_def into struct rv_monitor") moved the list_head, and this
> wrong type cast became a functional problem.
> 
> Properly use container_of() instead.

Good catch, thanks!

Reviewed-by: Gabriele Monaco <gmonaco@redhat.com>

> 
> Fixes: 24cbfe18d55a ("rv: Merge struct rv_monitor_def into struct
> rv_monitor")
> Signed-off-by: Nam Cao <namcao@linutronix.de>
> ---
>  kernel/trace/rv/rv.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/trace/rv/rv.c b/kernel/trace/rv/rv.c
> index bd7d56dbf6c2..6ce3495164d8 100644
> --- a/kernel/trace/rv/rv.c
> +++ b/kernel/trace/rv/rv.c
> @@ -495,7 +495,7 @@ static void *available_monitors_next(struct
> seq_file *m, void *p, loff_t *pos)
>   */
>  static void *enabled_monitors_next(struct seq_file *m, void *p,
> loff_t *pos)
>  {
> -	struct rv_monitor *mon = p;
> +	struct rv_monitor *mon = container_of(p, struct rv_monitor,
> list);
>  
>  	(*pos)++;
>  

Re: [PATCH] rv: Fix wrong type cast in enabled_monitors_next()

[Nathan Chancellor]

Hi Nam,

On Wed, Aug 06, 2025 at 02:09:11PM +0200, Nam Cao wrote:
> Argument 'p' of enabled_monitors_next() is not a pointer to struct
> rv_monitor, it is actually a pointer to the list_head inside struct
> rv_monitor. Therefore it is wrong to cast 'p' to struct rv_monitor *.
> 
> This wrong type cast has been there since the beginning. But it still
> worked because the list_head was the first field in struct rv_monitor_def.
> This is no longer true since commit 24cbfe18d55a ("rv: Merge struct
> rv_monitor_def into struct rv_monitor") moved the list_head, and this wrong
> type cast became a functional problem.
> 
> Properly use container_of() instead.
> 
> Fixes: 24cbfe18d55a ("rv: Merge struct rv_monitor_def into struct rv_monitor")
> Signed-off-by: Nam Cao <namcao@linutronix.de>
> ---
>  kernel/trace/rv/rv.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/trace/rv/rv.c b/kernel/trace/rv/rv.c
> index bd7d56dbf6c2..6ce3495164d8 100644
> --- a/kernel/trace/rv/rv.c
> +++ b/kernel/trace/rv/rv.c
> @@ -495,7 +495,7 @@ static void *available_monitors_next(struct seq_file *m, void *p, loff_t *pos)
>   */
>  static void *enabled_monitors_next(struct seq_file *m, void *p, loff_t *pos)
>  {
> -	struct rv_monitor *mon = p;
> +	struct rv_monitor *mon = container_of(p, struct rv_monitor, list);
>  
>  	(*pos)++;

I am seeing a crash when reading from /sys/kernel/tracing/rv/enabled_monitors
on a couple of my arm64 boxes running Fedora after this change, which
landed in mainline in 6.17-rc7. I can reproduce this in QEMU pretty
easily.

  $ cat kernel/configs/repro.config
  CONFIG_FTRACE=y
  CONFIG_RV=y
  CONFIG_RV_MON_WWNR=y
  CONFIG_RV_PER_TASK_MONITORS=2
  CONFIG_RV_REACTORS=y
  CONFIG_RV_REACT_PANIC=y
  CONFIG_RV_REACT_PRINTK=y

  $ git show -s --format='%h ("%s")'
  097a6c336d00 ("Merge tag 'trace-rv-v6.17-rc5' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace")

  $ make -skj"$(nproc)" ARCH=arm64 CROSS_COMPILE=aarch64-linux- clean virtconfig repro.config Image.gz

  $ curl -LSs https://github.com/ClangBuiltLinux/boot-utils/releases/download/20241120-044434/arm64-rootfs.cpio.zst | zstd -d >rootfs.cpio

  $ qemu-system-aarch64 \
      -display none \
      -nodefaults \
      -cpu max,pauth-impdef=true \
      -machine virt,gic-version=max,virtualization=true \
      -append 'console=ttyAMA0 earlycon rdinit=/bin/sh'
      -kernel arch/arm64/boot/Image.gz \
      -initrd rootfs.cpio \
      -m 512m \
      -serial mon:stdio
  ...
  [    0.000000] Linux version 6.17.0-rc6-00143-g097a6c336d00 (nathan@aadp) (aarch64-linux-gcc (GCC) 15.2.0, GNU ld (GNU Binutils) 2.45) #1 SMP PREEMPT Mon Sep 22 17:03:48 MST 2025
  ...
  ~ # mount -t sysfs sysfs /sys &&
  > mount -t tracefs tracing /sys/kernel/tracing &&
  > cat /sys/kernel/tracing/rv/enabled_monitors
  [  134.672494] Unable to handle kernel paging request at virtual address ffffffffffffffd0
  [  134.675919] Mem abort info:
  [  134.677233]   ESR = 0x0000000096000006
  [  134.679118]   EC = 0x25: DABT (current EL), IL = 32 bits
  [  134.680880]   SET = 0, FnV = 0
  [  134.682180]   EA = 0, S1PTW = 0
  [  134.683788]   FSC = 0x06: level 2 translation fault
  [  134.685435] Data abort info:
  [  134.687096]   ISV = 0, ISS = 0x00000006, ISS2 = 0x00000000
  [  134.688973]   CM = 0, WnR = 0, TnD = 0, TagAccess = 0
  [  134.690889]   GCS = 0, Overlay = 0, DirtyBit = 0, Xs = 0
  [  134.692687] swapper pgtable: 4k pages, 52-bit VAs, pgdp=00000000417a8000
  [  134.694969] [ffffffffffffffd0] pgd=1000000041c8d003, p4d=0000000041bf2403, pud=0000000041bf3403, pmd=0000000000000000
  [  134.700920] Internal error: Oops: 0000000096000006 [#1]  SMP
  [  134.702603] Modules linked in:
  [  134.703956] CPU: 0 UID: 0 PID: 53 Comm: cat Not tainted 6.17.0-rc6-00143-g097a6c336d00 #1 PREEMPT
  [  134.705506] Hardware name: linux,dummy-virt (DT)
  [  134.706481] pstate: a1402009 (NzCv daif +PAN -UAO -TCO +DIT -SSBS BTYPE=--)
  [  134.707595] pc : enabled_monitors_start+0x6c/0xa0
  [  134.709324] lr : enabled_monitors_start+0x24/0xa0
  [  134.710047] sp : ffff80008043bbf0
  [  134.710557] x29: ffff80008043bbf0 x28: fff0000002e58000 x27: 0000000000000000
  [  134.711889] x26: ffff80008043bca0 x25: fff0000002b44038 x24: 0000000000000000
  [  134.712965] x23: 0000000000000001 x22: 0000000000000000 x21: ffff80008043bcc8
  [  134.714028] x20: ffffa8bf4243e000 x19: ffffa8bf4243ed00 x18: 0000000000000000
  [  134.714994] x17: 0000000000000000 x16: 0000000000000000 x15: 0000000000000000
  [  134.716046] x14: 0000000000000000 x13: 0000000000000000 x12: 0000000000000000
  [  134.717110] x11: 0000000000000000 x10: 0000000000000000 x9 : 0000000000000000
  [  134.718219] x8 : 0000000000000000 x7 : 0000000000000001 x6 : 0000000000000000
  [  134.719246] x5 : 0000000000001000 x4 : fff000001fdea760 x3 : 0000000000000000
  [  134.720195] x2 : 0000000000000000 x1 : fff0000002b44028 x0 : ffffffffffffffc0
  [  134.721411] Call trace:
  [  134.722173]  enabled_monitors_start+0x6c/0xa0 (P)
  [  134.723088]  seq_read_iter+0xd4/0x47c
  [  134.723752]  seq_read+0xec/0x12c
  [  134.724200]  vfs_read+0xc4/0x33c
  [  134.724791]  ksys_read+0x64/0x100
  [  134.725255]  __arm64_sys_read+0x18/0x24
  [  134.725860]  invoke_syscall.constprop.0+0x40/0xf0
  [  134.726466]  el0_svc_common.constprop.0+0x38/0xd8
  [  134.727092]  do_el0_svc+0x1c/0x28
  [  134.727558]  el0_svc+0x34/0xe8
  [  134.728112]  el0t_64_sync_handler+0xa0/0xe4
  [  134.728763]  el0t_64_sync+0x198/0x19c
  [  134.729790] Code: f9402001 d1010020 eb01027f 540000c0 (39404001)
  [  134.731074] ---[ end trace 0000000000000000 ]---
  Segmentation fault

With this change reverted, there is no crash. As this change seems to
have proper justification, is there some other latent bug here?

If there is any other information I can provide or patches I can test, I
am more than happy to do so.

Cheers,
Nathan

Re: [PATCH] rv: Fix wrong type cast in enabled_monitors_next()

[Nam Cao]

Hi Nathan,

Nathan Chancellor <nathan@kernel.org> writes:
> I am seeing a crash when reading from /sys/kernel/tracing/rv/enabled_monitors
> on a couple of my arm64 boxes running Fedora after this change, which
> landed in mainline in 6.17-rc7. I can reproduce this in QEMU pretty
> easily.
...
> With this change reverted, there is no crash. As this change seems to
> have proper justification, is there some other latent bug here?

Thanks for the report.

Yes, this patch is broken, because argument 'p' of
enabled_monitors_next() *is* a pointer to struct rv_monitor. I'm not
sure how did I even test this patch... Steven is right, we really need
something in kselftest for RV, another thing in my RV TODO list.

But reverting is not the real fix, because monitors_show() still expects
a pointer to list_head. Changing monitors_show() is not an option,
because it is shared with the 'available_monitors' interface.

So the real fix is completely changing the iterator to be list_head
instead of rv_monitor.

Best regards,
Nam

diff --git a/kernel/trace/rv/rv.c b/kernel/trace/rv/rv.c
index 48338520376f..43e9ea473cda 100644
--- a/kernel/trace/rv/rv.c
+++ b/kernel/trace/rv/rv.c
@@ -501,7 +501,7 @@ static void *enabled_monitors_next(struct seq_file *m, void *p, loff_t *pos)
 
 	list_for_each_entry_continue(mon, &rv_monitors_list, list) {
 		if (mon->enabled)
-			return mon;
+			return &mon->list;
 	}
 
 	return NULL;
@@ -509,7 +509,7 @@ static void *enabled_monitors_next(struct seq_file *m, void *p, loff_t *pos)
 
 static void *enabled_monitors_start(struct seq_file *m, loff_t *pos)
 {
-	struct rv_monitor *mon;
+	struct list_head *head;
 	loff_t l;
 
 	mutex_lock(&rv_interface_lock);
@@ -517,15 +517,15 @@ static void *enabled_monitors_start(struct seq_file *m, loff_t *pos)
 	if (list_empty(&rv_monitors_list))
 		return NULL;
 
-	mon = list_entry(&rv_monitors_list, struct rv_monitor, list);
+	head = &rv_monitors_list;
 
 	for (l = 0; l <= *pos; ) {
-		mon = enabled_monitors_next(m, mon, &l);
-		if (!mon)
+		head = enabled_monitors_next(m, head, &l);
+		if (!head)
 			break;
 	}
 
-	return mon;
+	return head;
 }
 
 /*

Re: [PATCH] rv: Fix wrong type cast in enabled_monitors_next()

[Gabriele Monaco]

On Tue, 2025-09-23 at 07:28 +0200, Nam Cao wrote:
> Hi Nathan,
> 

Thanks for finding this!

> Nathan Chancellor <nathan@kernel.org> writes:
> > I am seeing a crash when reading from
> > /sys/kernel/tracing/rv/enabled_monitors
> > on a couple of my arm64 boxes running Fedora after this change, which
> > landed in mainline in 6.17-rc7. I can reproduce this in QEMU pretty
> > easily.
> ...
> > With this change reverted, there is no crash. As this change seems to
> > have proper justification, is there some other latent bug here?
> 
> Thanks for the report.
> 
> Yes, this patch is broken, because argument 'p' of
> enabled_monitors_next() *is* a pointer to struct rv_monitor. I'm not
> sure how did I even test this patch...

Damn, I'm wondering the same :facepalm: ..

> Steven is right, we really need something in kselftest for RV, another thing
> in my RV TODO list.

I can work on that, at least a few selftests for the sysfs, I think this gets
the top priority now.

> 
> But reverting is not the real fix, because monitors_show() still expects
> a pointer to list_head. Changing monitors_show() is not an option,
> because it is shared with the 'available_monitors' interface.
> 
> So the real fix is completely changing the iterator to be list_head
> instead of rv_monitor.

Looks reasonable, can you work on the fix?
I see Steve is out for conferences so this won't be too urgent.

Thanks,
Gabriele

> 
> Best regards,
> Nam
> 
> diff --git a/kernel/trace/rv/rv.c b/kernel/trace/rv/rv.c
> index 48338520376f..43e9ea473cda 100644
> --- a/kernel/trace/rv/rv.c
> +++ b/kernel/trace/rv/rv.c
> @@ -501,7 +501,7 @@ static void *enabled_monitors_next(struct seq_file *m,
> void *p, loff_t *pos)
>  
>  	list_for_each_entry_continue(mon, &rv_monitors_list, list) {
>  		if (mon->enabled)
> -			return mon;
> +			return &mon->list;
>  	}
>  
>  	return NULL;
> @@ -509,7 +509,7 @@ static void *enabled_monitors_next(struct seq_file *m,
> void *p, loff_t *pos)
>  
>  static void *enabled_monitors_start(struct seq_file *m, loff_t *pos)
>  {
> -	struct rv_monitor *mon;
> +	struct list_head *head;
>  	loff_t l;
>  
>  	mutex_lock(&rv_interface_lock);
> @@ -517,15 +517,15 @@ static void *enabled_monitors_start(struct seq_file *m,
> loff_t *pos)
>  	if (list_empty(&rv_monitors_list))
>  		return NULL;
>  
> -	mon = list_entry(&rv_monitors_list, struct rv_monitor, list);
> +	head = &rv_monitors_list;
>  
>  	for (l = 0; l <= *pos; ) {
> -		mon = enabled_monitors_next(m, mon, &l);
> -		if (!mon)
> +		head = enabled_monitors_next(m, head, &l);
> +		if (!head)
>  			break;
>  	}
>  
> -	return mon;
> +	return head;
>  }
>  
>  /*

Re: [PATCH] rv: Fix wrong type cast in enabled_monitors_next()

[Nam Cao]

Gabriele Monaco <gmonaco@redhat.com> writes:
> Looks reasonable, can you work on the fix?
> I see Steve is out for conferences so this won't be too urgent.

The fix is in my previous email.

I am guessing your email client is hiding the diff from you ;) (among
wrapping quoted reply, but that's not too important).

Nam

Re: [PATCH] rv: Fix wrong type cast in enabled_monitors_next()

[Gabriele Monaco]

On Tue, 2025-09-23 at 10:44 +0200, Nam Cao wrote:
> Gabriele Monaco <gmonaco@redhat.com> writes:
> > Looks reasonable, can you work on the fix?
> > I see Steve is out for conferences so this won't be too urgent.
> 
> The fix is in my previous email.
> 
> I am guessing your email client is hiding the diff from you ;) (among
> wrapping quoted reply, but that's not too important).

Uh, that's my brain ignoring what's under the signature, need to look for an
update to that..

Going to test it.

Thanks,
Gabriele

Re: [PATCH] rv: Fix wrong type cast in enabled_monitors_next()

[Gabriele Monaco]

On Tue, 2025-09-23 at 10:44 +0200, Nam Cao wrote:
> Gabriele Monaco <gmonaco@redhat.com> writes:
> > Looks reasonable, can you work on the fix?
> > I see Steve is out for conferences so this won't be too urgent.
> 
> The fix is in my previous email.

I confirm enabled_monitors works with your fix. Feel free to post a patch and
I'll battle test it and include it.

Thanks,
Gabriele

> 
> I am guessing your email client is hiding the diff from you ;) (among
> wrapping quoted reply, but that's not too important).
> 
> Nam