hppa: Revise gettext trampoline design

The current getcontext return trampoline is overly complex and it
unnecessarily clobbers several registers.  By saving the context
pointer (r26) in the context, __getcontext_ret can restore any
registers not restored by setcontext.  This allows getcontext to
save and restore the entire register context present when getcontext
is entered.  We use the unused oR0 context slot for the return
from __getcontext_ret.

While this is not directly useful in C, it can be exploited in
assembly code.  Registers r20, r23, r24 and r25 are not clobbered
in the call path to getcontext.  This allows a small simplification
of swapcontext.

It also allows saving and restoring the 6-bit SAR register in the
LSB of the oSAR context slot.  The getcontext flag value can be
stored in the MSB of the oSAR slot.
This commit is contained in:
John David Anglin 2022-02-22 17:28:46 +00:00
parent fdc1ae67fe
commit 9e7e5fda38
3 changed files with 35 additions and 31 deletions

View File

@ -21,22 +21,28 @@
#include "ucontext_i.h" #include "ucontext_i.h"
/* Trampoline function. Non-standard calling ABI. */ /* Trampoline function. Non-standard calling ABI. */
/* Can not use ENTRY(__getcontext_ret) here. */ /* Can not use ENTRY(__getcontext_ret) here. */
.type __getcontext_ret, @function .type __getcontext_ret, @function
.hidden __getcontext_ret .hidden __getcontext_ret
__getcontext_ret: __getcontext_ret:
.proc .proc
.callinfo FRAME=0,NO_CALLS .callinfo FRAME=0,NO_CALLS
/* r26-r23 contain original r3-r6, but because setcontext /* Because setcontext does not reload r3-r6 (it's using them
does not reload r3-r6 (it's using them as temporaries) as temporaries), we must load them ourself. */
we must save them elsewhere and swap them back in. */ ldw oR3(%r26), %r3
copy %r23, %r3 ldw oR4(%r26), %r4
copy %r24, %r4 ldw oR5(%r26), %r5
copy %r25, %r5 ldw oR6(%r26), %r6
copy %r26, %r6
/* r20 contains original return pointer. */ /* Also reload registers clobbered by $$dyncall. */
bv 0(%r20) ldw oR21(%r26), %r21
ldw oR22(%r26), %r22
ldw oR31(%r26), %r31
/* oR0 contains original return pointer. */
ldw oR0(%r26), %rp
bv 0(%rp)
copy %r0, %ret0 copy %r0, %ret0
.procend .procend
.size __getcontext_ret, .-__getcontext_ret .size __getcontext_ret, .-__getcontext_ret
@ -64,13 +70,13 @@ ENTRY(__getcontext)
stw %r17, oR17(%r26) stw %r17, oR17(%r26)
stw %r18, oR18(%r26) stw %r18, oR18(%r26)
stw %r19, oR19(%r26) stw %r19, oR19(%r26)
/* stw %r20, oR20(%r26) - used for trampoline. */ stw %r20, oR20(%r26)
stw %r21, oR21(%r26) stw %r21, oR21(%r26)
stw %r22, oR22(%r26) stw %r22, oR22(%r26)
/* stw %r23, oR23(%r26) - used for trampoline. */ stw %r23, oR23(%r26)
/* stw %r24, oR24(%r26) - used for trampoline. */ stw %r24, oR24(%r26)
/* stw %r25, oR25(%r26) - used for trampoline. */ stw %r25, oR25(%r26)
/* stw %r26, oR26(%r26) - used for trampoline. */ stw %r26, oR26(%r26)
stw %r27, oR27(%r26) stw %r27, oR27(%r26)
stw %r28, oR28(%r26) stw %r28, oR28(%r26)
stw %r29, oR29(%r26) stw %r29, oR29(%r26)
@ -89,7 +95,10 @@ ENTRY(__getcontext)
stw %r0, oIASQ1(%r26) stw %r0, oIASQ1(%r26)
stw %r0, oIAOQ0(%r26) stw %r0, oIAOQ0(%r26)
stw %r0, oIAOQ1(%r26) stw %r0, oIAOQ1(%r26)
stw %r0, oSAR(%r26) /* used as flag in swapcontext(). */
/* Save SAR register. */
mfctl %sar, %r1
stw %r1, oSAR(%r26) /* MSB used as flag in swapcontext(). */
/* Store floating-point regs. */ /* Store floating-point regs. */
@ -141,13 +150,8 @@ ENTRY(__getcontext)
.cfi_offset 29, 4 .cfi_offset 29, 4
/* Set up the trampoline registers. /* Set up the trampoline registers.
r20, r23, r24, r25, r26 and r2 are clobbered Use oR0 context slot to save return value. */
by call to getcontext() anyway. Reuse them. */ stw %r2, oR0(%r26)
stw %r2, oR20(%r26)
stw %r3, oR23(%r26)
stw %r4, oR24(%r26)
stw %r5, oR25(%r26)
stw %r6, oR26(%r26)
#ifdef PIC #ifdef PIC
addil LT%__getcontext_ret, %r19 addil LT%__getcontext_ret, %r19
ldw RT%__getcontext_ret(%r1), %r1 ldw RT%__getcontext_ret(%r1), %r1

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@ -75,7 +75,7 @@ ENTRY(__setcontext)
ldw oR18(%r3), %r18 ldw oR18(%r3), %r18
ldw oR19(%r3), %r19 ldw oR19(%r3), %r19
ldw oR20(%r3), %r20 ldw oR20(%r3), %r20
ldw oR21(%r3), %r21 ldw oR21(%r3), %r21 /* maybe clobbered by dyncall */
/* ldw oR22(%r3), %r22 - dyncall arg. */ /* ldw oR22(%r3), %r22 - dyncall arg. */
ldw oR23(%r3), %r23 ldw oR23(%r3), %r23
ldw oR24(%r3), %r24 ldw oR24(%r3), %r24
@ -87,6 +87,10 @@ ENTRY(__setcontext)
ldw oR30(%r3), %sp ldw oR30(%r3), %sp
/* ldw oR31(%r3), %r31 - dyncall scratch register */ /* ldw oR31(%r3), %r31 - dyncall scratch register */
/* Restore SAR register. */
ldw oSAR(%r3), %r22
mtsar %r22
/* Restore floating-point registers. */ /* Restore floating-point registers. */
ldo oFPREGS31(%r3), %r22 ldo oFPREGS31(%r3), %r22
fldds 0(%r22), %fr31 fldds 0(%r22), %fr31

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@ -25,10 +25,6 @@ extern int __setcontext (const ucontext_t *ucp);
int int
__swapcontext (ucontext_t *oucp, const ucontext_t *ucp) __swapcontext (ucontext_t *oucp, const ucontext_t *ucp)
{ {
/* Save ucp in stack argument slot. */
asm ("stw %r25,-40(%sp)");
asm (".cfi_offset 25, -40");
/* Save rp for debugger. */ /* Save rp for debugger. */
asm ("stw %rp,-20(%sp)"); asm ("stw %rp,-20(%sp)");
asm (".cfi_offset 2, -20"); asm (".cfi_offset 2, -20");
@ -59,7 +55,7 @@ __swapcontext (ucontext_t *oucp, const ucontext_t *ucp)
asm ("bv,n %r0(%rp)"); asm ("bv,n %r0(%rp)");
/* Load sc_sar flag. */ /* Load sc_sar flag. */
asm ("ldw %0(%%ret1),%%r20" : : "i" (oSAR)); asm ("ldb %0(%%ret1),%%r20" : : "i" (oSAR));
/* Return if oucp context has been reactivated. */ /* Return if oucp context has been reactivated. */
asm ("or,= %r0,%r20,%r0"); asm ("or,= %r0,%r20,%r0");
@ -67,11 +63,11 @@ __swapcontext (ucontext_t *oucp, const ucontext_t *ucp)
/* Mark sc_sar flag. */ /* Mark sc_sar flag. */
asm ("1: ldi 1,%r20"); asm ("1: ldi 1,%r20");
asm ("stw %%r20,%0(%%ret1)" : : "i" (oSAR)); asm ("stb %%r20,%0(%%ret1)" : : "i" (oSAR));
/* Activate the machine context in ucp. */ /* Activate the machine context in ucp. */
asm ("bl __setcontext,%rp"); asm ("bl __setcontext,%rp");
asm ("ldw -40(%sp),%r26"); asm ("ldw %0(%%ret1),%%r26" : : "i" (oR25));
/* Load return pointer. */ /* Load return pointer. */
asm ("ldw %0(%%ret1),%%rp" : : "i" (oR28)); asm ("ldw %0(%%ret1),%%rp" : : "i" (oR28));