[PATCHv2] sim: Don't overwrite stored errno in sim_syscall_multi

  I originally posted this patch back in February and figured it might
be worth giving it a refresh.  Here's the original submission, it
hasn't changed:

    https://sourceware.org/ml/gdb-patches/2018-02/msg00062.html

The sim/ maintainer has been rather absent of late, so I'm hoping
someone else might approve this.

I've been using this patch with an out-of-tree RISC-V simulator for
over a year now without any problems.

Thanks,
Andrew

---

The host syscall callback mechanism should take care of updating the
errcode within the CB_SYSCALL struct, and we should not be adjusting
the error code once the syscall has completed.  We especially, should
not be rewriting the syscall errcode based on the value of errno some
time after running the host syscall, as there is no guarantee that
errno has not be overwritten.

To perform a syscall we call cb_syscall (in syscall.c).  To return
from cb_syscall control passes through one of two exit paths these are
labeled FinishSyscall and ErrorFinish and are reached using goto
statements scattered throughout the cb_syscall function.

In FinishSyscall we store the syscall result in 'sc->result', and the
error code is transated to target encoding, and stored in
'sc->errcode'.

In ErrorFinish, we again store the syscall result in 'sc->result', and
fill in 'sc->errcode' by fetching the actual errno from the host with
the 'cb->get_errno' callback.

In both cases 'sc->errcode' will have been filled in with an
appropriate value.

Further, if we look at a specific syscall example, CB_SYS_open, in
this case the first thing we do is fetch the path to open from the
target with 'get_path', if this fails then the errcode is returned,
and we jump to FinishSyscall.  Notice that in this case, no host
syscall may have been performed, for example a failure to read the
path to open out of simulated memory can return EINVAL without
performing any host syscall.  Given that no host syscall has been
performed, reading the host errno makes absolutely no sense.

This commit removes from sim_syscall_multi the rewriting of
sc->errcode based on the value of errno, and instead relies on the
value stored in the cb_syscall.

sim/common/ChangeLog:

	* sim-syscall.c (sim_syscall_multi): Don't update sc->errcode at
	this point, it should have already been set in cb_syscall.
---
 sim/common/ChangeLog     | 5 +++++
 sim/common/sim-syscall.c | 5 -----
 2 files changed, 5 insertions(+), 5 deletions(-)
  

On 2018-12-12 18:41, Andrew Burgess wrote:
> I originally posted this patch back in February and figured it might
> be worth giving it a refresh.  Here's the original submission, it
> hasn't changed:
> 
>     https://sourceware.org/ml/gdb-patches/2018-02/msg00062.html
> 
> The sim/ maintainer has been rather absent of late, so I'm hoping
> someone else might approve this.
> 
> I've been using this patch with an out-of-tree RISC-V simulator for
> over a year now without any problems.
> 
> Thanks,
> Andrew
> 
> ---
> 
> The host syscall callback mechanism should take care of updating the
> errcode within the CB_SYSCALL struct, and we should not be adjusting
> the error code once the syscall has completed.  We especially, should
> not be rewriting the syscall errcode based on the value of errno some
> time after running the host syscall, as there is no guarantee that
> errno has not be overwritten.
> 
> To perform a syscall we call cb_syscall (in syscall.c).  To return
> from cb_syscall control passes through one of two exit paths these are
> labeled FinishSyscall and ErrorFinish and are reached using goto
> statements scattered throughout the cb_syscall function.
> 
> In FinishSyscall we store the syscall result in 'sc->result', and the
> error code is transated to target encoding, and stored in
> 'sc->errcode'.
> 
> In ErrorFinish, we again store the syscall result in 'sc->result', and
> fill in 'sc->errcode' by fetching the actual errno from the host with
> the 'cb->get_errno' callback.
> 
> In both cases 'sc->errcode' will have been filled in with an
> appropriate value.
> 
> Further, if we look at a specific syscall example, CB_SYS_open, in
> this case the first thing we do is fetch the path to open from the
> target with 'get_path', if this fails then the errcode is returned,
> and we jump to FinishSyscall.  Notice that in this case, no host
> syscall may have been performed, for example a failure to read the
> path to open out of simulated memory can return EINVAL without
> performing any host syscall.  Given that no host syscall has been
> performed, reading the host errno makes absolutely no sense.
> 
> This commit removes from sim_syscall_multi the rewriting of
> sc->errcode based on the value of errno, and instead relies on the
> value stored in the cb_syscall.
> 
> sim/common/ChangeLog:
> 
> 	* sim-syscall.c (sim_syscall_multi): Don't update sc->errcode at
> 	this point, it should have already been set in cb_syscall.
> ---
>  sim/common/ChangeLog     | 5 +++++
>  sim/common/sim-syscall.c | 5 -----
>  2 files changed, 5 insertions(+), 5 deletions(-)

Hi Andrew,

I took some time to familiarize myself with the issue, I think what you 
say make sense and that your patch is fine, please push.

I would have liked to try it, but none of the currently upstream arches 
that use sim_syscall_multi are easy for me to use (m32r, lm32, mn10300).

Simon