xtensa: fix fast_syscall_spill_registers_fixup

fast_syscall_spill_registers_fixup was not correctly updated by the
'keep a3 and excsave1 on entry to exception handlers' patch: it doesn't
preserve a3 that it gets on entry, breaking _spill_registers in case of
page fault on stack during register spilling, leading to unhandled
exception in kernel mode.

Preserve a3 by saving it in the original _spill_registers stack frame's
a3 during exception handling and restoring it afterwards.

Also fix comments and function bounds annotations.

Reported-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Tested-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Chris Zankel <chris@zankel.net>

1 files changed, 30 insertions, 19 deletions

diff --git a/arch/xtensa/kernel/entry.S b/arch/xtensa/kernel/entry.S
index de1dfa18d0a1..21dbe6bdb8ed 100644
--- a/arch/xtensa/kernel/entry.S
+++ b/arch/xtensa/kernel/entry.S
@@ -1122,7 +1122,7 @@ ENDPROC(fast_syscall_spill_registers)
  * a3: exctable, original value in excsave1
  */
 
-fast_syscall_spill_registers_fixup:
+ENTRY(fast_syscall_spill_registers_fixup)
 
         rsr     a2, windowbase  # get current windowbase (a2 is saved)
         xsr     a0, depc        # restore depc and a0
@@ -1134,22 +1134,26 @@ fast_syscall_spill_registers_fixup:
          */
 
         xsr     a3, excsave1    # get spill-mask
-        slli    a2, a3, 1       # shift left by one
+        slli    a3, a3, 1       # shift left by one
 
-        slli    a3, a2, 32-WSBITS
-        src     a2, a2, a3      # a1 = xxwww1yyxxxwww1yy......
+        slli    a2, a3, 32-WSBITS
+        src     a2, a3, a2      # a2 = xxwww1yyxxxwww1yy......
         wsr     a2, windowstart # set corrected windowstart
 
-        rsr     a3, excsave1
-        l32i    a2, a3, EXC_TABLE_DOUBLE_SAVE   # restore a2
-        l32i    a3, a3, EXC_TABLE_PARAM # original WB (in user task)
+        srli    a3, a3, 1
+        rsr     a2, excsave1
+        l32i    a2, a2, EXC_TABLE_DOUBLE_SAVE   # restore a2
+        xsr     a2, excsave1
+        s32i    a3, a2, EXC_TABLE_DOUBLE_SAVE   # save a3
+        l32i    a3, a2, EXC_TABLE_PARAM # original WB (in user task)
+        xsr     a2, excsave1
 
         /* Return to the original (user task) WINDOWBASE.
          * We leave the following frame behind:
          * a0, a1, a2   same
-         * a3:          trashed (saved in excsave_1)
+         * a3:          trashed (saved in EXC_TABLE_DOUBLE_SAVE)
          * depc:        depc (we have to return to that address)
-         * excsave_1:   a3
+         * excsave_1:   exctable
          */
 
         wsr     a3, windowbase
@@ -1159,9 +1163,9 @@ fast_syscall_spill_registers_fixup:
          *  a0: return address
          *  a1: used, stack pointer
          *  a2: kernel stack pointer
-         *  a3: available, saved in EXCSAVE_1
+         *  a3: available
          *  depc: exception address
-         *  excsave: a3
+         *  excsave: exctable
          * Note: This frame might be the same as above.
          */
 
@@ -1181,9 +1185,12 @@ fast_syscall_spill_registers_fixup:
         rsr     a0, exccause
         addx4   a0, a0, a3                      # find entry in table
         l32i    a0, a0, EXC_TABLE_FAST_USER     # load handler
+        l32i    a3, a3, EXC_TABLE_DOUBLE_SAVE
         jx      a0
 
-fast_syscall_spill_registers_fixup_return:
+ENDPROC(fast_syscall_spill_registers_fixup)
+
+ENTRY(fast_syscall_spill_registers_fixup_return)
 
         /* When we return here, all registers have been restored (a2: DEPC) */
 
@@ -1191,13 +1198,13 @@ fast_syscall_spill_registers_fixup_return:
 
         /* Restore fixup handler. */
 
-        xsr     a3, excsave1
-        movi    a2, fast_syscall_spill_registers_fixup
-        s32i    a2, a3, EXC_TABLE_FIXUP
-        s32i    a0, a3, EXC_TABLE_DOUBLE_SAVE
-        rsr     a2, windowbase
-        s32i    a2, a3, EXC_TABLE_PARAM
-        l32i    a2, a3, EXC_TABLE_KSTK
+        rsr     a2, excsave1
+        s32i    a3, a2, EXC_TABLE_DOUBLE_SAVE
+        movi    a3, fast_syscall_spill_registers_fixup
+        s32i    a3, a2, EXC_TABLE_FIXUP
+        rsr     a3, windowbase
+        s32i    a3, a2, EXC_TABLE_PARAM
+        l32i    a2, a2, EXC_TABLE_KSTK
 
         /* Load WB at the time the exception occurred. */
 
@@ -1206,8 +1213,12 @@ fast_syscall_spill_registers_fixup_return:
         wsr     a3, windowbase
         rsync
 
+        rsr     a3, excsave1
+        l32i    a3, a3, EXC_TABLE_DOUBLE_SAVE
+
         rfde
 
+ENDPROC(fast_syscall_spill_registers_fixup_return)
 
 /*
  * spill all registers.