• David S. Miller's avatar
    sparc64: Fix return from trap window fill crashes. · 7cafc0b8
    David S. Miller authored
    We must handle data access exception as well as memory address unaligned
    exceptions from return from trap window fill faults, not just normal
    TLB misses.
    
    Otherwise we can get an OOPS that looks like this:
    
    ld-linux.so.2(36808): Kernel bad sw trap 5 [#1]
    CPU: 1 PID: 36808 Comm: ld-linux.so.2 Not tainted 4.6.0 #34
    task: fff8000303be5c60 ti: fff8000301344000 task.ti: fff8000301344000
    TSTATE: 0000004410001601 TPC: 0000000000a1a784 TNPC: 0000000000a1a788 Y: 00000002    Not tainted
    TPC: <do_sparc64_fault+0x5c4/0x700>
    g0: fff8000024fc8248 g1: 0000000000db04dc g2: 0000000000000000 g3: 0000000000000001
    g4: fff8000303be5c60 g5: fff800030e672000 g6: fff8000301344000 g7: 0000000000000001
    o0: 0000000000b95ee8 o1: 000000000000012b o2: 0000000000000000 o3: 0000000200b9b358
    o4: 0000000000000000 o5: fff8000301344040 sp: fff80003013475c1 ret_pc: 0000000000a1a77c
    RPC: <do_sparc64_fault+0x5bc/0x700>
    l0: 00000000000007ff l1: 0000000000000000 l2: 000000000000005f l3: 0000000000000000
    l4: fff8000301347e98 l5: fff8000024ff3060 l6: 0000000000000000 l7: 0000000000000000
    i0: fff8000301347f60 i1: 0000000000102400 i2: 0000000000000000 i3: 0000000000000000
    i4: 0000000000000000 i5: 0000000000000000 i6: fff80003013476a1 i7: 0000000000404d4c
    I7: <user_rtt_fill_fixup+0x6c/0x7c>
    Call Trace:
     [0000000000404d4c] user_rtt_fill_fixup+0x6c/0x7c
    
    The window trap handlers are slightly clever, the trap table entries for them are
    composed of two pieces of code.  First comes the code that actually performs
    the window fill or spill trap handling, and then there are three instructions at
    the end which are for exception processing.
    
    The userland register window fill handler is:
    
    	add	%sp, STACK_BIAS + 0x00, %g1;		\
    	ldxa	[%g1 + %g0] ASI, %l0;			\
    	mov	0x08, %g2;				\
    	mov	0x10, %g3;				\
    	ldxa	[%g1 + %g2] ASI, %l1;			\
    	mov	0x18, %g5;				\
    	ldxa	[%g1 + %g3] ASI, %l2;			\
    	ldxa	[%g1 + %g5] ASI, %l3;			\
    	add	%g1, 0x20, %g1;				\
    	ldxa	[%g1 + %g0] ASI, %l4;			\
    	ldxa	[%g1 + %g2] ASI, %l5;			\
    	ldxa	[%g1 + %g3] ASI, %l6;			\
    	ldxa	[%g1 + %g5] ASI, %l7;			\
    	add	%g1, 0x20, %g1;				\
    	ldxa	[%g1 + %g0] ASI, %i0;			\
    	ldxa	[%g1 + %g2] ASI, %i1;			\
    	ldxa	[%g1 + %g3] ASI, %i2;			\
    	ldxa	[%g1 + %g5] ASI, %i3;			\
    	add	%g1, 0x20, %g1;				\
    	ldxa	[%g1 + %g0] ASI, %i4;			\
    	ldxa	[%g1 + %g2] ASI, %i5;			\
    	ldxa	[%g1 + %g3] ASI, %i6;			\
    	ldxa	[%g1 + %g5] ASI, %i7;			\
    	restored;					\
    	retry; nop; nop; nop; nop;			\
    	b,a,pt	%xcc, fill_fixup_dax;			\
    	b,a,pt	%xcc, fill_fixup_mna;			\
    	b,a,pt	%xcc, fill_fixup;
    
    And the way this works is that if any of those memory accesses
    generate an exception, the exception handler can revector to one of
    those final three branch instructions depending upon which kind of
    exception the memory access took.  In this way, the fault handler
    doesn't have to know if it was a spill or a fill that it's handling
    the fault for.  It just always branches to the last instruction in
    the parent trap's handler.
    
    For example, for a regular fault, the code goes:
    
    winfix_trampoline:
    	rdpr	%tpc, %g3
    	or	%g3, 0x7c, %g3
    	wrpr	%g3, %tnpc
    	done
    
    All window trap handlers are 0x80 aligned, so if we "or" 0x7c into the
    trap time program counter, we'll get that final instruction in the
    trap handler.
    
    On return from trap, we have to pull the register window in but we do
    this by hand instead of just executing a "restore" instruction for
    several reasons.  The largest being that from Niagara and onward we
    simply don't have enough levels in the trap stack to fully resolve all
    possible exception cases of a window fault when we are already at
    trap level 1 (which we enter to get ready to return from the original
    trap).
    
    This is executed inline via the FILL_*_RTRAP handlers.  rtrap_64.S's
    code branches directly to these to do the window fill by hand if
    necessary.  Now if you look at them, we'll see at the end:
    
    	    ba,a,pt    %xcc, user_rtt_fill_fixup;
    	    ba,a,pt    %xcc, user_rtt_fill_fixup;
    	    ba,a,pt    %xcc, user_rtt_fill_fixup;
    
    And oops, all three cases are handled like a fault.
    
    This doesn't work because each of these trap types (data access
    exception, memory address unaligned, and faults) store their auxiliary
    info in different registers to pass on to the C handler which does the
    real work.
    
    So in the case where the stack was unaligned, the unaligned trap
    handler sets up the arg registers one way, and then we branched to
    the fault handler which expects them setup another way.
    
    So the FAULT_TYPE_* value ends up basically being garbage, and
    randomly would generate the backtrace seen above.
    Reported-by: default avatarNick Alcock <nix@esperi.org.uk>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    7cafc0b8
rtrap_64.S 8.8 KB