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Fix makecontext() on MIPS O32.

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The GP register can be clobbered by the callback, so save it in S1
while invoking the callback function.

While here, add a comment expounding on the treatment of GP for the
various ABIs and the assumptions made.

Reviewed by:	jmallett (earlier version)
Sponsored by:	DARPA / AFRL
Differential Revision:	https://reviews.freebsd.org/D14179
This commit is contained in:
John Baldwin 2018-02-05 18:10:28 +00:00
parent 7dea788b91
commit 0b2b653012
1 changed files with 15 additions and 1 deletions
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16
lib/libc/mips/gen/_ctx_start.S
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@ -28,11 +28,25 @@
__FBSDID("$FreeBSD$");
/*
* XXX gp?
* This requires makecontext() to setup a valid GP for locating
* _ctx_done rather than deriving GP from T9 on entry. Currently this
* uses the GP inherited from getcontext() assuming that getcontext()
* is in the same shared object as _ctx_done(). For N32 and N64, GP
* is caller-save so will be preserved across the call to the callback
* function. For O32, GP is callee-save, so save it in a different
* caller-save register (S1) while invoking the callback. This is
* done instead of the usual SETUP_GP/SAVE_GP to avoid disturbing the
* stack frame setup by makecontext() for the callback function.
*/
ENTRY(_ctx_start)
#ifdef __mips_o32
move s1, gp
#endif
jalr t9
#ifdef __mips_o32
move gp, s1
#endif
move a0, s0
PTR_LA t9, _ctx_done
jalr t9