Re: Kernel Oops on alpha with kernel version >=6.9.x

[Ivan Kokshaysky <ink@unseen.parts>]

On Sat, Jan 25, 2025 at 06:59:32PM +0000, Maciej W. Rozycki wrote:
> On Sat, 25 Jan 2025, Maciej W. Rozycki wrote:
> 
> > > Interesting. Perhaps these frames are aligned by PAL-code as well,
> > > the reference manual wasn't clear about that.
> > 
> >  I think it just boils down to the amount of exception nesting.
> 
>  Ah, actually most of these faults were entered from the user mode and the 
> kernel stack starts at a page boundary, so once the stack frame has been 
> allocated by PALcode and SAVE_ALL combined the stack pointer ends up 
> misaligned.  For faults entered from the kernel mode the opposite might be 
> the case.

Indeed, sounds plausible.

> So unless we want to play (and we don't) with FP and the saving 
> and restoration of SP, we just want to keep SP aligned at all times.

> > > I knew about entUna, I thought it's safe as it only deals with 64-bit data
> > > and not going to be changed in future, but missed entMM...
> > > 
> > > I agree, better fix both.
> > 
> >  Well, we may get away with it in many cases, which is obviously why 
> > this bug has survived so long, but in principle it is not safe to enter 
> > C code with the stack misaligned, so yes, we need to fix all the code 
> > paths, also because a nested exception will cause hell to break loose.  
> > 
> >  Here just bumping up the frame size and adjusting offsets in assembly 
> > code accordingly so as to account for the empty longword at the bottom 
> > of the frame should do, just as I did across my change.
> 
>  ... or, depending on how you look at it, top of the frame and FAOD in any 
> case the longword closest to the stack pointer will be the empty one.

Right. So if we agree on my variant, this addition patch is needed.

[No problems with gdb, as expected.]

Ivan.

diff --git a/arch/alpha/kernel/entry.S b/arch/alpha/kernel/entry.S
index 6fb38365539d..f4d41b4538c2 100644
--- a/arch/alpha/kernel/entry.S
+++ b/arch/alpha/kernel/entry.S
@@ -194,8 +194,8 @@ CFI_END_OSF_FRAME entArith
 CFI_START_OSF_FRAME entMM
 	SAVE_ALL
 /* save $9 - $15 so the inline exception code can manipulate them.  */
-	subq	$sp, 56, $sp
-	.cfi_adjust_cfa_offset	56
+	subq	$sp, 64, $sp
+	.cfi_adjust_cfa_offset	64
 	stq	$9, 0($sp)
 	stq	$10, 8($sp)
 	stq	$11, 16($sp)
@@ -210,7 +210,7 @@ CFI_START_OSF_FRAME entMM
 	.cfi_rel_offset	$13, 32
 	.cfi_rel_offset	$14, 40
 	.cfi_rel_offset	$15, 48
-	addq	$sp, 56, $19
+	addq	$sp, 64, $19
 /* handle the fault */
 	lda	$8, 0x3fff
 	bic	$sp, $8, $8
@@ -223,7 +223,7 @@ CFI_START_OSF_FRAME entMM
 	ldq	$13, 32($sp)
 	ldq	$14, 40($sp)
 	ldq	$15, 48($sp)
-	addq	$sp, 56, $sp
+	addq	$sp, 64, $sp
 	.cfi_restore	$9
 	.cfi_restore	$10
 	.cfi_restore	$11
@@ -231,7 +231,7 @@ CFI_START_OSF_FRAME entMM
 	.cfi_restore	$13
 	.cfi_restore	$14
 	.cfi_restore	$15
-	.cfi_adjust_cfa_offset	-56
+	.cfi_adjust_cfa_offset	-64
 /* finish up the syscall as normal.  */
 	br	ret_from_sys_call
 CFI_END_OSF_FRAME entMM
@@ -378,8 +378,8 @@ entUnaUser:
 	.cfi_restore	$0
 	.cfi_adjust_cfa_offset	-256
 	SAVE_ALL		/* setup normal kernel stack */
-	lda	$sp, -56($sp)
-	.cfi_adjust_cfa_offset	56
+	lda	$sp, -64($sp)
+	.cfi_adjust_cfa_offset	64
 	stq	$9, 0($sp)
 	stq	$10, 8($sp)
 	stq	$11, 16($sp)
@@ -395,7 +395,7 @@ entUnaUser:
 	.cfi_rel_offset	$14, 40
 	.cfi_rel_offset	$15, 48
 	lda	$8, 0x3fff
-	addq	$sp, 56, $19
+	addq	$sp, 64, $19
 	bic	$sp, $8, $8
 	jsr	$26, do_entUnaUser
 	ldq	$9, 0($sp)
@@ -405,7 +405,7 @@ entUnaUser:
 	ldq	$13, 32($sp)
 	ldq	$14, 40($sp)
 	ldq	$15, 48($sp)
-	lda	$sp, 56($sp)
+	lda	$sp, 64($sp)
 	.cfi_restore	$9
 	.cfi_restore	$10
 	.cfi_restore	$11
@@ -413,7 +413,7 @@ entUnaUser:
 	.cfi_restore	$13
 	.cfi_restore	$14
 	.cfi_restore	$15
-	.cfi_adjust_cfa_offset	-56
+	.cfi_adjust_cfa_offset	-64
 	br	ret_from_sys_call
 CFI_END_OSF_FRAME entUna