* [PATCH] powerpc/64s: Don't use DSISR for SLB faults
@ 2022-02-22 11:34 Michael Ellerman
2022-02-23 6:46 ` Nicholas Piggin
2022-03-08 12:07 ` Michael Ellerman
0 siblings, 2 replies; 3+ messages in thread
From: Michael Ellerman @ 2022-02-22 11:34 UTC (permalink / raw)
To: linuxppc-dev; +Cc: dja, rnsastry
Since commit 46ddcb3950a2 ("powerpc/mm: Show if a bad page fault on data
is read or write.") we use page_fault_is_write(regs->dsisr) in
__bad_page_fault() to determine if the fault is for a read or write, and
change the message printed accordingly.
But SLB faults, aka Data Segment Interrupts, don't set DSISR (Data
Storage Interrupt Status Register) to a useful value. All ISA versions
from v2.03 through v3.1 specify that the Data Segment Interrupt sets
DSISR "to an undefined value". As far as I can see there's no mention of
SLB faults setting DSISR in any BookIV content either.
This manifests as accesses that should be a read being incorrectly
reported as writes, for example, using the xmon "dump" command:
0:mon> d 0x5deadbeef0000000
5deadbeef0000000
[359526.415354][ C6] BUG: Unable to handle kernel data access on write at 0x5deadbeef0000000
[359526.415611][ C6] Faulting instruction address: 0xc00000000010a300
cpu 0x6: Vector: 380 (Data SLB Access) at [c00000000ffbf400]
pc: c00000000010a300: mread+0x90/0x190
If we disassemble the PC, we see a load instruction:
0:mon> di c00000000010a300
c00000000010a300 89490000 lbz r10,0(r9)
We can also see in exceptions-64s.S that the data_access_slb block
doesn't set IDSISR=1, which means it doesn't load DSISR into pt_regs. So
the value we're using to determine if the fault is a read/write is some
stale value in pt_regs from a previous page fault.
Rework the printing logic to separate the SLB fault case out, and only
print read/write in the cases where we can determine it.
The result looks like eg:
0:mon> d 0x5deadbeef0000000
5deadbeef0000000
[ 721.779525][ C6] BUG: Unable to handle kernel data access at 0x5deadbeef0000000
[ 721.779697][ C6] Faulting instruction address: 0xc00000000014cbe0
cpu 0x6: Vector: 380 (Data SLB Access) at [c00000000ffbf390]
0:mon> d 0
0000000000000000
[ 742.793242][ C6] BUG: Kernel NULL pointer dereference at 0x00000000
[ 742.793316][ C6] Faulting instruction address: 0xc00000000014cbe0
cpu 0x6: Vector: 380 (Data SLB Access) at [c00000000ffbf390]
Fixes: 46ddcb3950a2 ("powerpc/mm: Show if a bad page fault on data is read or write.")
Reported-by: Nageswara R Sastry <rnsastry@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
---
arch/powerpc/mm/fault.c | 14 ++++++++++----
1 file changed, 10 insertions(+), 4 deletions(-)
diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
index eb8ecd7343a9..7ba6d3eff636 100644
--- a/arch/powerpc/mm/fault.c
+++ b/arch/powerpc/mm/fault.c
@@ -567,18 +567,24 @@ NOKPROBE_SYMBOL(hash__do_page_fault);
static void __bad_page_fault(struct pt_regs *regs, int sig)
{
int is_write = page_fault_is_write(regs->dsisr);
+ const char *msg;
/* kernel has accessed a bad area */
+ if (regs->dar < PAGE_SIZE)
+ msg = "Kernel NULL pointer dereference";
+ else
+ msg = "Unable to handle kernel data access";
+
switch (TRAP(regs)) {
case INTERRUPT_DATA_STORAGE:
- case INTERRUPT_DATA_SEGMENT:
case INTERRUPT_H_DATA_STORAGE:
- pr_alert("BUG: %s on %s at 0x%08lx\n",
- regs->dar < PAGE_SIZE ? "Kernel NULL pointer dereference" :
- "Unable to handle kernel data access",
+ pr_alert("BUG: %s on %s at 0x%08lx\n", msg,
is_write ? "write" : "read", regs->dar);
break;
+ case INTERRUPT_DATA_SEGMENT:
+ pr_alert("BUG: %s at 0x%08lx\n", msg, regs->dar);
+ break;
case INTERRUPT_INST_STORAGE:
case INTERRUPT_INST_SEGMENT:
pr_alert("BUG: Unable to handle kernel instruction fetch%s",
--
2.34.1
^ permalink raw reply related [flat|nested] 3+ messages in thread* Re: [PATCH] powerpc/64s: Don't use DSISR for SLB faults
2022-02-22 11:34 [PATCH] powerpc/64s: Don't use DSISR for SLB faults Michael Ellerman
@ 2022-02-23 6:46 ` Nicholas Piggin
2022-03-08 12:07 ` Michael Ellerman
1 sibling, 0 replies; 3+ messages in thread
From: Nicholas Piggin @ 2022-02-23 6:46 UTC (permalink / raw)
To: linuxppc-dev, Michael Ellerman; +Cc: rnsastry, dja
Excerpts from Michael Ellerman's message of February 22, 2022 9:34 pm:
> Since commit 46ddcb3950a2 ("powerpc/mm: Show if a bad page fault on data
> is read or write.") we use page_fault_is_write(regs->dsisr) in
> __bad_page_fault() to determine if the fault is for a read or write, and
> change the message printed accordingly.
>
> But SLB faults, aka Data Segment Interrupts, don't set DSISR (Data
> Storage Interrupt Status Register) to a useful value. All ISA versions
> from v2.03 through v3.1 specify that the Data Segment Interrupt sets
> DSISR "to an undefined value". As far as I can see there's no mention of
> SLB faults setting DSISR in any BookIV content either.
>
> This manifests as accesses that should be a read being incorrectly
> reported as writes, for example, using the xmon "dump" command:
>
> 0:mon> d 0x5deadbeef0000000
> 5deadbeef0000000
> [359526.415354][ C6] BUG: Unable to handle kernel data access on write at 0x5deadbeef0000000
> [359526.415611][ C6] Faulting instruction address: 0xc00000000010a300
> cpu 0x6: Vector: 380 (Data SLB Access) at [c00000000ffbf400]
> pc: c00000000010a300: mread+0x90/0x190
>
> If we disassemble the PC, we see a load instruction:
>
> 0:mon> di c00000000010a300
> c00000000010a300 89490000 lbz r10,0(r9)
>
> We can also see in exceptions-64s.S that the data_access_slb block
> doesn't set IDSISR=1, which means it doesn't load DSISR into pt_regs. So
> the value we're using to determine if the fault is a read/write is some
> stale value in pt_regs from a previous page fault.
>
> Rework the printing logic to separate the SLB fault case out, and only
> print read/write in the cases where we can determine it.
>
> The result looks like eg:
>
> 0:mon> d 0x5deadbeef0000000
> 5deadbeef0000000
> [ 721.779525][ C6] BUG: Unable to handle kernel data access at 0x5deadbeef0000000
> [ 721.779697][ C6] Faulting instruction address: 0xc00000000014cbe0
> cpu 0x6: Vector: 380 (Data SLB Access) at [c00000000ffbf390]
>
> 0:mon> d 0
> 0000000000000000
> [ 742.793242][ C6] BUG: Kernel NULL pointer dereference at 0x00000000
> [ 742.793316][ C6] Faulting instruction address: 0xc00000000014cbe0
> cpu 0x6: Vector: 380 (Data SLB Access) at [c00000000ffbf390]
>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
> Fixes: 46ddcb3950a2 ("powerpc/mm: Show if a bad page fault on data is read or write.")
> Reported-by: Nageswara R Sastry <rnsastry@linux.ibm.com>
> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
> ---
> arch/powerpc/mm/fault.c | 14 ++++++++++----
> 1 file changed, 10 insertions(+), 4 deletions(-)
>
> diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
> index eb8ecd7343a9..7ba6d3eff636 100644
> --- a/arch/powerpc/mm/fault.c
> +++ b/arch/powerpc/mm/fault.c
> @@ -567,18 +567,24 @@ NOKPROBE_SYMBOL(hash__do_page_fault);
> static void __bad_page_fault(struct pt_regs *regs, int sig)
> {
> int is_write = page_fault_is_write(regs->dsisr);
> + const char *msg;
>
> /* kernel has accessed a bad area */
>
> + if (regs->dar < PAGE_SIZE)
> + msg = "Kernel NULL pointer dereference";
> + else
> + msg = "Unable to handle kernel data access";
> +
> switch (TRAP(regs)) {
> case INTERRUPT_DATA_STORAGE:
> - case INTERRUPT_DATA_SEGMENT:
> case INTERRUPT_H_DATA_STORAGE:
> - pr_alert("BUG: %s on %s at 0x%08lx\n",
> - regs->dar < PAGE_SIZE ? "Kernel NULL pointer dereference" :
> - "Unable to handle kernel data access",
> + pr_alert("BUG: %s on %s at 0x%08lx\n", msg,
> is_write ? "write" : "read", regs->dar);
> break;
> + case INTERRUPT_DATA_SEGMENT:
> + pr_alert("BUG: %s at 0x%08lx\n", msg, regs->dar);
> + break;
> case INTERRUPT_INST_STORAGE:
> case INTERRUPT_INST_SEGMENT:
> pr_alert("BUG: Unable to handle kernel instruction fetch%s",
> --
> 2.34.1
>
>
^ permalink raw reply [flat|nested] 3+ messages in thread* Re: [PATCH] powerpc/64s: Don't use DSISR for SLB faults
2022-02-22 11:34 [PATCH] powerpc/64s: Don't use DSISR for SLB faults Michael Ellerman
2022-02-23 6:46 ` Nicholas Piggin
@ 2022-03-08 12:07 ` Michael Ellerman
1 sibling, 0 replies; 3+ messages in thread
From: Michael Ellerman @ 2022-03-08 12:07 UTC (permalink / raw)
To: Michael Ellerman, linuxppc-dev; +Cc: dja, rnsastry
On Tue, 22 Feb 2022 22:34:49 +1100, Michael Ellerman wrote:
> Since commit 46ddcb3950a2 ("powerpc/mm: Show if a bad page fault on data
> is read or write.") we use page_fault_is_write(regs->dsisr) in
> __bad_page_fault() to determine if the fault is for a read or write, and
> change the message printed accordingly.
>
> But SLB faults, aka Data Segment Interrupts, don't set DSISR (Data
> Storage Interrupt Status Register) to a useful value. All ISA versions
> from v2.03 through v3.1 specify that the Data Segment Interrupt sets
> DSISR "to an undefined value". As far as I can see there's no mention of
> SLB faults setting DSISR in any BookIV content either.
>
> [...]
Applied to powerpc/next.
[1/1] powerpc/64s: Don't use DSISR for SLB faults
https://git.kernel.org/powerpc/c/d4679ac8ea2e5078704aa1c026db36580cc1bf9a
cheers
^ permalink raw reply [flat|nested] 3+ messages in thread
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2022-02-22 11:34 [PATCH] powerpc/64s: Don't use DSISR for SLB faults Michael Ellerman
2022-02-23 6:46 ` Nicholas Piggin
2022-03-08 12:07 ` Michael Ellerman
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