Rewrite sigaction() to prevent another race.
A signal handler might call sigaction(). We must block all signals, not only our signal, to prevent a race between us and the next signal handler. Use /* comments */ because cpp might expand macros in ! comments though such expansion is probably harmless. The bridge is now shorter by 2 instructions.
This commit is contained in:
George Koehler
parent
2b09d3756c
commit
103d44c27c
1 changed files with 161 additions and 123 deletions
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@ -1,156 +1,194 @@
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#define __NR_sigaction 67
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#define SIG_BLOCK 0
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#define __NR_sigprocmask 126
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#define SIG_SETMASK 2
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#define MAXSIG 32
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/* offsets into our stack frame */
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#define mynew 16 /* new sigaction */
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#define mynset 32 /* new signal set */
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#define myoset 36 /* old signal set */
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#define mysave 40
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#define mysize 56
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/* offsets into struct sigaction */
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#define sa_handler 0 /* in union with sa_sigaction */
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#define sa_mask 4
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#define sa_flags 8
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#define sa_restorer 12
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/* offsets from stack pointer */
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#define mynewact 16 /* struct sigaction */
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#define myoldact 32
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#define newmask 64 /* signal set */
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#define oldmask 68
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#define oldhandler 72
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#define myret 76
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#define savelr 80
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#define signum 84 /* first argument */
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#define newact 88
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#define oldact 92
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.sect .text; .sect .rodata; .sect .data; .sect .bss
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/*
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* Linux calls signal handlers with arguments in registers, but the
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* ACK expects arguments on the stack. This sigaction() uses a
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* "bridge" to move the arguments.
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* "bridge" to move the arguments, but
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*
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* - If the caller passes a bad pointer, this sigaction() causes
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* SIGBUS or SIGSEGV instead of setting errno = EFAULT.
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*
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* - This sigaction() only works with signals 1 to 31, not with
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* real-time signals 32 to 64.
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*
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* - This sigaction() is not safe for multiple threads.
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*
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* int sigaction(int signum, const struct sigaction *newact,
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* struct sigaction *oldact);
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*/
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.sect .text
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.define _sigaction
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_sigaction:
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mflr r0
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subi r1, r1, mysize
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stw r31, mysave+8(r1)
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stw r30, mysave+4(r1)
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stw r29, mysave(r1)
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stw r0, mysave+12(r1)
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li r3, 0
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stw r3, mynset(r1) ! mynset = 0
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lwz r29, mysize(r1) ! r29 = signal number
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lwz r30, mysize+4(r1) ! r30 = new action
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lwz r31, mysize+8(r1) ! r31 = old action
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li r3, __NR_sigprocmask
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stwu r3, -signum(sp) /* keep 0(sp) = __NR_sigprocmask */
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stw r0, savelr(sp)
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/* Copy newact to stack (before blocking SIGBUS, SIGSEGV). */
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lwz r3, newact(sp)
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mr. r3, r3
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beq 1f /* skip if newact == NULL */
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lwz r4, sa_handler(r3)
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lwz r5, sa_mask(r3)
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lwz r6, sa_flags(r3)
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lwz r7, sa_restorer(r3)
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stw r4, mynewact+sa_handler(sp)
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stw r5, mynewact+sa_mask(sp)
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stw r6, mynewact+sa_flags(sp)
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stw r7, mynewact+sa_restorer(sp)
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/*
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* If the new action is non-NULL, the signal number is in
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* range 1 to MAXSIG, and the new handler is not SIG_DFL 0
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* or SIG_IGN 1, then we interpose our bridge.
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* Block all signals to prevent a race. After we set sharray,
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* we must call the kernel's sigaction before the next signal
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* handler runs. This prevents two problems:
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*
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* - The bridge might call the new handler while the kernel
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* uses the mask and flags of the old handler.
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*
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* - The signal handler might call sigaction() and destroy
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* sharray. We must block all signals because any signal
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* handler might call sigaction() for our signal.
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*/
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cmpwi cr0, r30, 0
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subi r7, r29, 1 ! r7 = index in handlers
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cmplwi cr7, r7, MAXSIG ! unsigned comparison
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beq cr0, kernel
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bge cr7, kernel
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lwz r3, 0(r30) ! r3 = new handler
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clrrwi. r3, r3, 1
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beq cr0, kernel
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/*
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* Block the signal while we build the bridge. Prevents a
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* race if a signal arrives after we change the bridge but
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* before we change the action in the kernel.
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*/
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li r4, 1
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slw r4, r4, r7
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stw r4, mynset(r1) ! mynmask = 1 << (signal - 1)
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li r3, SIG_BLOCK
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la r4, mynset(r1)
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la r5, myoset(r1)
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stw r3, 0(r1)
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stw r4, 4(r1)
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stw r5, 8(r1)
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bl _sigprocmask
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/*
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* Point our bridge to the new signal handler. Then copy the
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* new sigaction but point it to our bridge.
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*/
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lis r6, hi16[handlers]
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ori r6, r6, lo16[handlers]
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subi r7, r29, 1
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slwi r7, r7, 2
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lwz r3, 0(r30) ! r3 = new handler
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stwx r3, r6, r7 ! put it in array of handlers
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lis r3, hi16[bridge]
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ori r3, r3, lo16[bridge]
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lwz r4, 4(r30)
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lwz r5, 8(r30)
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lwz r6, 12(r30)
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stw r3, mynew(r1) ! sa_handler or sa_sigaction
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stw r4, mynew+4(r1) ! sa_mask
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stw r5, mynew+8(r1) ! sa_flags
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stw r6, mynew+12(r1) ! sa_restorer
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la r30, mynew(r1)
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kernel:
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li r3, __NR_sigaction
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stw r3, 0(r1)
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stw r29, 4(r1)
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stw r30, 8(r1)
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stw r31, 12(r1)
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1: li r4, SIG_SETMASK
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li r5, -1 /* mask signals 1 to 32 */
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stw r5, newmask(sp)
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la r5, newmask(sp)
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la r6, oldmask(sp)
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stw r4, 4(sp) /* kept 0(sp) = __NR_sigprocmask */
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stw r5, 8(sp)
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stw r6, 12(sp)
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bl __syscall
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/*
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* If we blocked the signal, then restore the old signal mask.
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* If the signal number is in range 1 to 31, and the new
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* handler is not SIG_DFL 0 or SIG_IGN 1, then we interpose
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* our bridge.
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*/
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lwz r3, mynset(r1)
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cmpwi cr0, r3, 0
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beq cr0, fixold
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li r3, SIG_SETMASK
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la r4, myoset(r1)
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li r5, 0
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stw r3, 0(r1)
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stw r4, 4(r1)
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stw r5, 8(r1)
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bl _sigprocmask
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/*
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* If the old sigaction is non-NULL and points to our bridge,
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* then point it to the signal handler.
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*/
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fixold:
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cmpwi cr0, r31, 0
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beq cr0, leave
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lis r3, hi16[bridge]
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ori r3, r3, lo16[bridge]
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lwz r4, 0(r31)
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cmpw cr0, r3, r4
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bne cr0, leave
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lis r6, hi16[handlers]
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ori r6, r6, lo16[handlers]
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subi r7, r29, 1
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slwi r7, r7, 2
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lwzx r3, r6, r7 ! get it from array of handlers
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stw r3, 0(r31) ! put it in old sigaction
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leave:
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lwz r0, mysave+12(r1)
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lwz r29, mysave(r1)
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lwz r30, mysave+4(r1)
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lwz r31, mysave+8(r1)
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addi r1, r1, mysize
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lwz r4, signum(sp) /* keep r4 = signum */
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addi r5, r4, -1
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cmplwi r5, 30
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bgt 2f /* skip if out of range */
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slwi r5, r5, 2 /* r5 = sharray index */
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lis r6, ha16[sharray]
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la r6, lo16[sharray](r6) /* r6 = sharray */
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lwzx r0, r6, r5
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stw r0, oldhandler(sp) /* remember old handler */
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lwz r0, newact(sp)
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mr. r0, r0
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beq 2f /* skip if newact == NULL */
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lwz r3, mynewact+sa_handler(sp)
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cmplwi r3, 2 /* r3 = new handler */
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blt 2f /* skip if SIG_DFL or SIG_IGN */
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stwx r3, r6, r5 /* put new handler in sharray */
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lis r3, ha16[sigbridge]
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la r3, lo16[sigbridge](r3)
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stw r3, mynewact+sa_handler(sp)
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/* Call the kernel's sigaction. */
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/* sigaction(signum, &mynewact or NULL, &myoldact or NULL) */
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2: li r3, __NR_sigaction
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lwz r0, newact(sp)
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mr. r0, r0
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beq 3f
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la r5, mynewact(sp)
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b 4f
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3: li r5, 0
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4: lwz r0, oldact(sp)
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mr. r0, r0
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beq 5f
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la r6, myoldact(sp)
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b 6f
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5: li r6, 0
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6: stw r3, 0(sp)
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stw r4, 4(sp) /* kept r4 = signum */
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stw r5, 8(sp)
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stw r6, 12(sp)
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bl __syscall
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stw r3, myret(sp)
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/* Unblock signals by restoring old signal mask. */
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li r3, __NR_sigprocmask
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li r4, SIG_SETMASK
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la r5, oldmask(sp)
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li r6, 0
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stw r3, 0(sp)
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stw r4, 4(sp)
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stw r5, 8(sp)
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stw r6, 12(sp)
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bl __syscall
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/* Copy oldact from stack (after unblocking BUS, SEGV). */
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lwz r3, oldact(sp)
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mr. r3, r3
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beq 8f /* skip if oldact == NULL */
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lwz r4, myoldact+sa_handler(sp)
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lis r5, ha16[sigbridge]
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la r5, lo16[sigbridge](r5)
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cmpw r4, r5
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bne 7f
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lwz r4, oldhandler(sp)
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7: lwz r5, myoldact+sa_mask(sp)
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lwz r6, myoldact+sa_flags(sp)
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lwz r7, myoldact+sa_restorer(sp)
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stw r4, sa_handler(r3)
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stw r5, sa_mask(r3)
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stw r6, sa_flags(r3)
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stw r7, sa_restorer(r3)
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8: lwz r0, savelr(sp)
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lwz r3, myret(sp)
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addi sp, sp, signum
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mtlr r0
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blr ! return from sigaction
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blr
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/*
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* Linux calls bridge(signum) or bridge(signum, info, context) with
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* arguments in registers r3, r4, r5.
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* Linux calls sigbridge(signum) or sigbridge(signum, info, context)
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* with arguments in registers r3, r4, r5.
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*/
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bridge:
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sigbridge:
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mflr r0
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subi r1, r1, 16
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stwu r3, -16(sp) /* signal number */
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stw r4, 4(sp) /* info */
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stw r5, 8(sp) /* context */
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stw r0, 12(r1)
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stw r3, 0(r1) ! signal number
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stw r4, 4(r1) ! info
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stw r5, 8(r1) ! context
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lis r6, hi16[handlers]
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ori r6, r6, lo16[handlers]
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subi r7, r3, 1
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slwi r7, r7, 2
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lis r6, hi16[sharray - 4]
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la r6, lo16[sharray - 4](r6)
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slwi r7, r3, 2
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lwzx r6, r6, r7
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mtctr r6
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bctrl ! call our signal handler
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bctrl /* call our signal handler */
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lwz r0, 12(r1)
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lwz r0, 12(sp)
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addi r1, r1, 16
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mtlr r0
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blr ! return from bridge
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blr /* sigreturn(2) */
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.sect .bss
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handlers:
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.space 4 * MAXSIG ! array of signal handlers
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sharray:
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.space 4 * 31 /* handlers for signals 1 to 31 */
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