[RFC 0/4] linux-user: simplify safe signal handling

[Warner Losh]

This is a quick RFC to see if something like this is worth doing.

I've created a new interface host_signal_set_pc. This allows us to move all the
nearly identical copies of rewind_if_in_safe_syscall into signal.c.  This
reduces the amount of code that needs to be rewritten for bsd-user's adaptation
of both the safe signal handling and the sigsegv/sigbus changes that have
happened. Since BSD's mcontext_t differs in some cases, we wouldn't be able to
share this between platforms, but it reduces the number of nearly identical
routines I'd have to write.

In addition, the assembler glue for the safe system calls is almost identical
between linux and bsd-user's fork. The only difference is inverting the system
call return to comply with the -ERRNO convention *-user uses in the rest of the
code which is native to Linux, but differs for the BSDs and other traditional
unix targets.

I know the patches may not be sliced and diced in the typical desired
fashion. This is a RFC, and the changes are short enough to be easily digested
though since it's quite repetitive.

These were extracted from the 'blitz' branch we have in the bsd-user fork and
then that was adapted to use the common code. I've pushed a branch to gitlab
(viewable at https://gitlab.com/bsdimp/qemu/-/tree/blitz if you prefer that to
fetching) that shows how these will be used. I'm working on upstreaming
bsd-user/signal.c next which will take a little bit of time to work into a place
where it can be reviewed here. I wanted to get feedback because this is
one chunk I can cleave off and make landing that easier.

Warner Losh (4):
  linux-user: Add host_signal_set_pc to set pc in mcontext
  linux-user/signal.c: Create a common rewind_if_in_safe_syscall
  linux-user/safe-syscall.inc.S: Move to common-user
  common-user: Allow return codes to be adjusted after sytsem call

 .../host/aarch64/safe-syscall.inc.S           |  1 +
 .../host/arm/safe-syscall.inc.S               |  1 +
 .../host/i386/safe-syscall.inc.S              |  1 +
 .../host/ppc64/safe-syscall.inc.S             |  1 +
 .../host/riscv/safe-syscall.inc.S             |  1 +
 .../host/s390x/safe-syscall.inc.S             |  1 +
 .../host/x86_64/safe-syscall.inc.S            |  1 +
 linux-user/host/aarch64/host-signal.h         |  5 +++++
 linux-user/host/aarch64/hostdep.h             | 20 -------------------
 linux-user/host/alpha/host-signal.h           |  5 +++++
 linux-user/host/arm/host-signal.h             |  5 +++++
 linux-user/host/arm/hostdep.h                 | 20 -------------------
 linux-user/host/i386/host-signal.h            |  5 +++++
 linux-user/host/i386/hostdep.h                | 20 -------------------
 linux-user/host/mips/host-signal.h            |  5 +++++
 linux-user/host/ppc/host-signal.h             |  5 +++++
 linux-user/host/ppc64/hostdep.h               | 20 -------------------
 linux-user/host/riscv/host-signal.h           |  5 +++++
 linux-user/host/riscv/hostdep.h               | 20 -------------------
 linux-user/host/s390/host-signal.h            |  5 +++++
 linux-user/host/s390x/hostdep.h               | 20 -------------------
 linux-user/host/sparc/host-signal.h           |  9 +++++++++
 linux-user/host/x86_64/host-signal.h          |  5 +++++
 linux-user/host/x86_64/hostdep.h              | 20 -------------------
 linux-user/safe-syscall.S                     |  1 +
 linux-user/signal.c                           | 18 ++++++++++++++++-
 meson.build                                   |  1 +
 27 files changed, 80 insertions(+), 141 deletions(-)
 rename {linux-user => common-user}/host/aarch64/safe-syscall.inc.S (99%)
 rename {linux-user => common-user}/host/arm/safe-syscall.inc.S (99%)
 rename {linux-user => common-user}/host/i386/safe-syscall.inc.S (99%)
 rename {linux-user => common-user}/host/ppc64/safe-syscall.inc.S (99%)
 rename {linux-user => common-user}/host/riscv/safe-syscall.inc.S (99%)
 rename {linux-user => common-user}/host/s390x/safe-syscall.inc.S (99%)
 rename {linux-user => common-user}/host/x86_64/safe-syscall.inc.S (99%)

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2.33.0