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Implement support for POSIX thread cancellations

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This change makes some miracle modifications to the System Five system
call support, which lets us have safe, correct, and atomic handling of
thread cancellations. It all turned out to be cheaper than anticipated
because it wasn't necessary to modify the system call veneers. We were
able to encode the cancellability of each system call into the magnums
found in libc/sysv/syscalls.sh. Since cancellations are so waq, we are
also supporting a lovely Musl Libc mask feature for raising ECANCELED.
This commit is contained in:
Justine Tunney 2022-11-04 01:04:43 -07:00
parent 37d40e087f
commit 2278327eba
No known key found for this signature in database
GPG key ID: BE714B4575D6E328
145 changed files with 715 additions and 265 deletions
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77
libc/sysv/systemfive.S
View file

@ -21,6 +21,7 @@
#include "libc/macros.internal.h"
#include "libc/nexgen32e/macros.h"
#include "libc/sysv/consts/sig.h"
#include "libc/thread/posixthread.internal.h"
#include "libc/sysv/consts/nr.h"
#define SIG_IGN 1
@ -107,19 +108,60 @@ __pid: .quad 0
.endobj __pid,globl,hidden
.previous
.privileged
systemfive_cancellable: # our pthread_cancel() miracle code
cmpb $0,__tls_enabled(%rip) # inspired by the musl libc design!
je 1f # we handle linux and bsd together!
mov %fs:0,%r10 # CosmoTib::tib_self
mov 0x28(%r10),%r10 # CosmoTib::tib_pthread
test %r10,%r10 # is it a posix thread?
jz 1f # it's spawn() probably
testb $PT_NOCANCEL,0x00(%r10) # PosixThread::flags
jnz 1f # canceler no cancelling
cmp $0,0x04(%r10) # PosixThread::cancelled
jne _pthread_cancel_sys # tail call for masked mode
.weak _pthread_cancel_sys # must be linked if we're cancelled
1: mov %rcx,%r10 # move the fourth argument
clc # no cancellable system calls exist
syscall # that have 7+ args on the bsd OSes
systemfive_cancellable_end: # i/o calls park here for long time
jc systemfive_errno
jmp 0f
jc 3f # we're now out of the limbo state!
1: cmp $-4095,%rax # but we still check again on eintr
jae 2f
ret # done if the system call succeeded
2: neg %eax # now examine the nature of failure
3: cmp EINTR(%rip),%eax # did SIGCANCEL cancel our i/o call
jne systemfive_errno # werent interrupted by OnSigCancel
cmpb $0,__tls_enabled(%rip) # make sure it's safe to grab %fs:0
je systemfive_errno # tls is disabled we can't continue
mov %fs:0,%rcx # CosmoTib::tib_self
mov 0x28(%rcx),%rcx # CosmoTib::tib_pthread
test %rcx,%rcx # is it a posix thread?
jz systemfive_errno # it's spawn() probably
testb $PT_NOCANCEL,0x00(%rcx) # PosixThread::flags
jnz systemfive_errno # cancellation is disabled
cmp $0,0x04(%rcx) # PosixThread::cancelled
je systemfive_errno # we aren't actually cancelled
jmp _pthread_cancel_sys # now we are in fact cancelled
.endfn systemfive_cancellable,globl,hidden
.globl systemfive_cancellable_end
.hidden systemfive_cancellable_end
.Lanchorpoint:
#if SupportsLinux() || SupportsMetal()
systemfive_linux:
and $0xfff,%eax
cmp $0xfff,%eax
and $0xfff,%eax # remove nonlinux bits from ordinal
cmp $0xfff,%eax # checks if unsupported by platform
je systemfive_enosys # never taken branches cost nothing
btr $11,%eax # 0x800 means a call is cancellable
jc systemfive_cancellable # it is handled by the holiest code
mov %rcx,%r10 # syscall instruction clobbers %rcx
push %rbp # linux never reads args from stack
mov %rsp,%rbp # having frame will help backtraces
syscall # this is known as a context switch
pop %rbp # next we check to see if it failed
cmp $-4095,%rax # system five nexgen32e abi § a.2.1
0: cmp $-4095,%rax # system five nexgen32e abi § a.2.1
jae systemfive_error # encodes errno as neg return value
ret
.endfn systemfive_linux,globl,hidden
@ -167,17 +209,10 @@ systemfive_bsdscrub:
systemfive_bsd:
cmp $0xfff,%ax
je systemfive_enosys
btr $11,%eax # checks/reset the 800 cancellable bit
jc systemfive_cancellable
mov %rcx,%r10
push %rbx
push 32(%rsp)
push 24(%rsp)
push 16(%rsp)
mov %rax,%rbx # save ordinal for SIGSYS crash report
syscall # bsd will need arg on stack sometimes
pop %rbx
pop %rbx
pop %rbx
pop %rbx
jc systemfive_errno # bsd sets carry flag if %rax is errno
ret
.endfn systemfive_bsd
@ -391,22 +426,6 @@ _init_systemfive_sigsys:
pop %rdi
1: .endfn _init_systemfive_sigsys
#endif
#if SupportsSystemv() && !defined(TINY)
_init_systemfive_syscall:
mov __NR_msyscall,%eax # syscall origin protect
cmp $0xfff,%ax # openbsd is pretty cool
jae _init_systemfive_done
push %rdi
push %rsi
.weak __privileged_addr
.weak __privileged_size
mov $__privileged_addr,%edi
mov $__privileged_size,%esi
syscall
pop %rsi
pop %rdi
// 𝑠𝑙𝑖𝑑𝑒
#endif /* TINY */
_init_systemfive_done:
nop
.init.end 300,_init_systemfive,globl,hidden