[Bug go/113143] New: Remove usage of ucontext.h

[Bug go/113143] New: Remove usage of ucontext.h

[kallisti5 at unixzen dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113143

            Bug ID: 113143
           Summary: Remove usage of ucontext.h
           Product: gcc
           Version: 13.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: go
          Assignee: ian at airs dot com
          Reporter: kallisti5 at unixzen dot com
  Target Milestone: ---

libgo/sysinfo.c references #include <ucontext.h> which breaks the libgo build
on strict POSIX compliance platforms.

ucontext.h was removed in issue 7, and types were migrated to signal.h.

Old 2004 spec:
https://pubs.opengroup.org/onlinepubs/009695399/basedefs/ucontext.h.html


Pretty much any recent spec: (ucontext.h is gone)
https://pubs.opengroup.org/onlinepubs/9699919799/

[Bug go/113143] Remove usage of ucontext.h

[kallisti5 at unixzen dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113143

--- Comment #1 from Alexander von Gluck <kallisti5 at unixzen dot com> ---
$ grep -R ucontext.h gcc/libgo

gcc/libgo/configure:#include <ucontext.h>
gcc/libgo/configure.ac:#include <ucontext.h>
gcc/libgo/go/runtime/runtime2.go:// _sizeof_ucontext_t is defined by
mkrsysinfo.sh from <ucontext.h>.
gcc/libgo/misc/cgo/testsanitizers/testdata/tsan_shared.go:#include <ucontext.h>
gcc/libgo/runtime/go-signal.c:#include <ucontext.h>
gcc/libgo/runtime/runtime.h:#include <ucontext.h>
gcc/libgo/sysinfo.c:#include <ucontext.h>
gcc/libgo/sysinfo.c:// From sys/ucontext.h


pretty much all of these references can be replaced with a signal.h include...
(or wrapped with HAVE_U_CONTEXT_H or whatever if concerned about backwards
compatibility to posix 2004.

[Bug go/113143] Remove usage of ucontext.h

[ian at airs dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113143

--- Comment #2 from Ian Lance Taylor <ian at airs dot com> ---
Note that except on x86_64 GNU/Linux, libgo requires the makecontext,
getcontext, and setcontext functions that are declared in ucontext.h.  There is
no adequate replacement for those functions on strict POSIX systems.  (On
x86_64 GNU/Linux libgo provides assembler versions of those functions (in
libgo/runtime/go-context.S) that are more efficient for libgo's purposes.)

[Bug go/113143] Remove usage of ucontext.h

[kallisti5 at unixzen dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113143

--- Comment #3 from Alexander von Gluck <kallisti5 at unixzen dot com> ---
Good call out.  It looks like in 2004 these functions were flagged as
obsolescent though:
https://pubs.opengroup.org/onlinepubs/009695399/functions/makecontext.html

Here's the note on it in the linux man page for getcontext:
"
  glibc 2.1.  SUSv2, POSIX.1-2001.  Removed in POSIX.1-2008, citing
  portability issues, and recommending that applications be
  rewritten to use POSIX threads instead.
"


upstream go usage:

* makecontext: no code matches as of go 1.22 (main)
* getcontext:  netbsd,solaris,freebsd code usage as of go 1.22 (main)
* setcontext:  netbsd,freebsd code usages as of go 1.22 (main)
* swapcontext: freebsd code usages as of go 1.22 (main)


Per the 2004 spec:

"The obsolescent functions getcontext(), makecontext(), and swapcontext() can
be replaced using POSIX threads functions."

Essentially, for quite some time, (get|make|set)context are only used on the
BSD's and Solaris... thus invocation of include<ucontext.h> should be guarded /
masked off on all other platforms.

[Bug go/113143] Remove usage of ucontext.h

[ian at airs dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113143

--- Comment #4 from Ian Lance Taylor <ian at airs dot com> ---
I understand that makecontext/getcontext/setcontext were removed from POSIX.
However, there is no adequate replacement.  Their functionality is absolutely
required for what libgo does.  Fortunately GNU/Linux systems continue to
provide them.

[Bug go/113143] Remove usage of ucontext.h

[kallisti5 at unixzen dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113143

--- Comment #5 from Alexander von Gluck <kallisti5 at unixzen dot com> ---
> I understand that makecontext/getcontext/setcontext were removed from POSIX. However, there is no adequate replacement.  Their functionality is absolutely required for what libgo does.

No Linux-specific code calls these in gccgo, or upstream go.  Only FreeBSD,
Solaris, and OpenBSD.

Thus my question, why is ucontext.h referenced in cross-platform code when
signal.h will suffice?

[Bug go/113143] Remove usage of ucontext.h

[kallisti5 at unixzen dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113143

--- Comment #6 from Alexander von Gluck <kallisti5 at unixzen dot com> ---
For added clairity. Searching the upstream go sources:

```
 grep -R getcontext
src/cmd/vendor/golang.org/x/sys/unix/syscall_netbsd.go:// getcontext
src/cmd/vendor/golang.org/x/sys/unix/zsysnum_freebsd_386.go:    SYS_GETCONTEXT 
             = 421 // { int getcontext(struct __ucontext *ucp); }
src/cmd/vendor/golang.org/x/sys/unix/zsysnum_freebsd_amd64.go:  SYS_GETCONTEXT 
             = 421 // { int getcontext(struct __ucontext *ucp); }
src/cmd/vendor/golang.org/x/sys/unix/zsysnum_freebsd_arm.go:    SYS_GETCONTEXT 
             = 421 // { int getcontext(struct __ucontext *ucp); }
src/cmd/vendor/golang.org/x/sys/unix/zsysnum_freebsd_arm64.go:  SYS_GETCONTEXT 
             = 421 // { int getcontext(struct __ucontext *ucp); }
src/cmd/vendor/golang.org/x/sys/unix/zsysnum_netbsd_386.go:     SYS_GETCONTEXT 
         = 307 // { int|sys||getcontext(struct __ucontext *ucp); }
src/cmd/vendor/golang.org/x/sys/unix/zsysnum_netbsd_amd64.go:   SYS_GETCONTEXT 
         = 307 // { int|sys||getcontext(struct __ucontext *ucp); }
src/cmd/vendor/golang.org/x/sys/unix/zsysnum_netbsd_arm.go:     SYS_GETCONTEXT 
         = 307 // { int|sys||getcontext(struct __ucontext *ucp); }
src/cmd/vendor/golang.org/x/sys/unix/zsysnum_netbsd_arm64.go:   SYS_GETCONTEXT 
         = 307 // { int|sys||getcontext(struct __ucontext *ucp); }
src/runtime/cgo/gcc_solaris_amd64.c:    if (getcontext(&ctx) != 0)
src/runtime/cgo/gcc_solaris_amd64.c:            perror("runtime/cgo: getcontext
failed");
src/runtime/os3_solaris.go://go:cgo_import_dynamic libc_getcontext getcontext
"libc.so"
src/runtime/os3_solaris.go://go:linkname libc_getcontext libc_getcontext
src/runtime/os3_solaris.go:     libc_getcontext,
src/runtime/os3_solaris.go:func getcontext(context *ucontext) /* int32 */ {
src/runtime/os3_solaris.go:     sysvicall1(&libc_getcontext,
uintptr(unsafe.Pointer(context)))
src/runtime/os_netbsd.go:func getcontext(ctxt unsafe.Pointer)
src/runtime/os_netbsd.go:       getcontext(unsafe.Pointer(&uc))
src/runtime/sys_netbsd_386.s:#define SYS_getcontext                     307
src/runtime/sys_netbsd_386.s:TEXT runtime·getcontext(SB),NOSPLIT,$-4
src/runtime/sys_netbsd_386.s:   MOVL    $SYS_getcontext, AX
src/runtime/sys_netbsd_amd64.s:#define SYS_getcontext                   307
src/runtime/sys_netbsd_amd64.s:TEXT runtime·getcontext(SB),NOSPLIT,$-8
src/runtime/sys_netbsd_amd64.s: MOVL    $SYS_getcontext, AX
src/runtime/sys_netbsd_arm.s:#define SYS_getcontext                    
SWI_OS_NETBSD | 307
src/runtime/sys_netbsd_arm.s:TEXT runtime·getcontext(SB),NOSPLIT|NOFRAME,$0
src/runtime/sys_netbsd_arm.s:   SWI $SYS_getcontext
src/runtime/sys_netbsd_arm64.s:#define SYS_getcontext                   307
src/runtime/sys_netbsd_arm64.s:TEXT runtime·getcontext(SB),NOSPLIT,$-8
src/runtime/sys_netbsd_arm64.s: SVC     $SYS_getcontext
src/syscall/zsysnum_freebsd_386.go:     SYS_GETCONTEXT               = 421 // {
int getcontext(struct __ucontext *ucp); }
src/syscall/zsysnum_freebsd_amd64.go:   SYS_GETCONTEXT               = 421 // {
int getcontext(struct __ucontext *ucp); }
src/syscall/zsysnum_freebsd_arm.go:     SYS_GETCONTEXT               = 421 // {
int getcontext(struct __ucontext *ucp); }
src/syscall/zsysnum_freebsd_arm64.go:   SYS_GETCONTEXT               = 421 // {
int getcontext(struct __ucontext *ucp); }
src/syscall/zsysnum_netbsd_386.go:      SYS_GETCONTEXT           = 307 // {
int|sys||getcontext(struct __ucontext *ucp); }
src/syscall/zsysnum_netbsd_amd64.go:    SYS_GETCONTEXT           = 307 // {
int|sys||getcontext(struct __ucontext *ucp); }
src/syscall/zsysnum_netbsd_arm.go:      SYS_GETCONTEXT           = 307 // {
int|sys||getcontext(struct __ucontext *ucp); }
src/syscall/zsysnum_netbsd_arm64.go:    SYS_GETCONTEXT           = 307 // {
int|sys||getcontext(struct __ucontext *ucp); }
```

Those all appear to be BSD-specific references, unless i'm missing an
invocation of (get|set|swap)context somewhere?

[Bug go/113143] Remove usage of ucontext.h

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113143

--- Comment #7 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Let me ask a dumb question, are you porting GCC/gccgo to a libc which does not
have ucontext.h ?  If so what is the libc my I ask?

[Bug go/113143] Remove usage of ucontext.h

[kallisti5 at unixzen dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113143

--- Comment #8 from Alexander von Gluck <kallisti5 at unixzen dot com> ---
Not at all a dumb question :-)

I'm working through compiling libgo / gccgo on Haiku.    We have our own lovely
libc which tests posix compliant.

Some historic consideration behind us not having ucontext.h is here:
https://dev.haiku-os.org/ticket/5324

We have around 3000 ports (including an old (c)go 1.4, rust, gcc, ruby, etc)
none of which have complained about a missing ucontext.h.  This is kinda why I
feel like it's an anti-pattern to require it in libgo.

Even Google's go doesn't consider ucontext.h a requirement on non-bsd
platforms.


Here's the guys of our libc for context:
https://cgit.haiku-os.org/haiku/tree/headers/posix

[Bug go/113143] Remove usage of ucontext.h

[kallisti5 at unixzen dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113143

--- Comment #9 from Alexander von Gluck <kallisti5 at unixzen dot com> ---
Doing a little research, it looks like complaints of ucontext.h have come up
before multiple times:

Similar issue on OpenBSD a long time ago:
https://gcc-help.gcc.gnu.narkive.com/Xx1bResV/can-t-build-go-support-in-gcc-4-8-1-on-openbsd

muslc not supporting ucontext.h:
https://wiki.musl-libc.org/open-issues

(and a long list of google results)


I guess the root questions of this bug are:

1. Should any and all references to ucontext.h be wrapped with a HAVE macro?
(this seems to be what most folks do).   Since (get/set/swap)context don't seem
to be called on non-BSD platforms, this seems possible?

2. Should libgo double down and implement processor-specific calls for each
architecture as called out by Ian in the above thread in 2013?

"It's possible to fix this by writing processor-dependent functions
that would serve the same purpose as the *context functions, and in
fact that would be more efficient. But I have not actually done this."

3. Is the expectation that all platforms which libgo targets include ucontext.h
compatibility libraries for calls no-longer standard in the posix
specifications since 2008.   (muslc pushed against this... i'd be interested if
libgo compiles under musl systems)

[Bug go/113143] Remove usage of ucontext.h

[ian at airs dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113143

--- Comment #10 from Ian Lance Taylor <ian at airs dot com> ---
The makecontext/getcontext/setcontext functions are called by libgo on
GNU/Linux for all targets other than x86_64.  It is not the case that they are
used only on *BSD systems.

The default gc implementation does not require these functions because it
provides assembler code.  This of course limits the gc implementation to only
support a limited number of targets: those for which the assembler code exists.
 libgo is intended to be more portable.

To be clear I am fine with removing or guarding references to ucontext.h where
that makes sense.  I'm just pointing out that the code can't in general work if
the *context functions are not available.  It's unfortunate that POSIX removed
them without providing adequate replacements.  Saying that they "can be
replaced using POSIX threads functions" is simply false.  They provide a
portable mechanism for a userspace threads mechanism on top of POSIX threads. 
Without them there is no way for libgo to implement goroutines efficiently.

[Bug go/113143] Remove usage of ucontext.h

[kallisti5 at unixzen dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113143

--- Comment #11 from Alexander von Gluck <kallisti5 at unixzen dot com> ---
Hm.. Could you point me to where Linux is using the (get/set/swap)context
calls?  I still haven't found them in the codebase.

I don't want to miss any edge cases if I work up a patch.

[Bug go/113143] Remove usage of ucontext.h

[ian at airs dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113143

--- Comment #12 from Ian Lance Taylor <ian at airs dot com> ---
libgo/runtime/runtime.h:

#if defined(__x86_64__) && defined(__linux__) && !defined(__CET__)
...
#else
#define __go_context_t  ucontext_t
#define __go_getcontext(c)      getcontext(c)
#define __go_setcontext(c)      setcontext(c)
#define __go_makecontext(c, fn, sp, size) \
        ((c)->uc_stack.ss_sp = sp, (c)->uc_stack.ss_size = size, makecontext(c,
fn, 0))
#endif


libgo/runtime/proc.c:

various calls to __go_getcontext, __go_setcontext, and __go_makecontext.