On Wed, 10 Sep 2003, Bjorn Helgaas wrote:
> Here's a 2.4 backport of this change to 2.5:
>
> http://linux.bkbits.net:8080/linux-2.5/[email protected]?nav=index.html
>
> Alpha, ppc, and sparc64 define force_successful_syscall_return() in 2.5,
> but since it's not obvious to me how to do it correctly in 2.4, I left
> them unchanged.
Whats the reasoning behing this patch?
On Wednesday 17 September 2003 2:00 pm, Marcelo Tosatti wrote:
>
> On Wed, 10 Sep 2003, Bjorn Helgaas wrote:
>
> > Here's a 2.4 backport of this change to 2.5:
> >
> > http://linux.bkbits.net:8080/linux-2.5/[email protected]?nav=index.html
> >
> > Alpha, ppc, and sparc64 define force_successful_syscall_return() in 2.5,
> > but since it's not obvious to me how to do it correctly in 2.4, I left
> > them unchanged.
>
> Whats the reasoning behing this patch?
Basically we don't want a large unsigned return value to be
misinterpreted as a syscall failure because it looks like
a small negative number.
>From David's description of the 2.5 patch (the link above has
the explanation):
Many architectures (alpha, ia64, ppc, ppc64, sparc, and sparc64 at least)
use a syscall convention which provides for a return value and a separate
error flag. On those architectures, it can be beneficial if the kernel
provides a mechanism to signal that a syscall call has completed
successfully, even when the returned value is potentially a (small)
negative number. The patch below provides a hook for such a mechanism via
a macro called force_successful_syscall_return(). On x86, this would be
simply a no-op (because on x86, user-level has to be hacked to handle such
cases).
On Thu, 2003-09-18 at 01:30, Marcelo Tosatti wrote:
> On Wed, 10 Sep 2003, Bjorn Helgaas wrote:
>
> > Here's a 2.4 backport of this change to 2.5:
> >
> >
> http://linux.bkbits.net:8080/linux-2.5/[email protected]?nav=index.html
> >
> > Alpha, ppc, and sparc64 define force_successful_syscall_return() in
> 2.5,
> > but since it's not obvious to me how to do it correctly in 2.4, I left
> > them unchanged.
>
> Whats the reasoning behing this patch?
IIRC those changes were added to 2.5 by David. Architecture like Ia64
and Alpha support error return via a different register set ( $19 for
Alpha ). But syscalls like ptrace can have negative return value for
successful returns. So in that particular case $19 is forced to be zero
to indicate it is a successful return. IIUC
force_successful_syscall_return is a wrapper around doing that. On
alpha actually r0 in the stack (regs.r0 ) is made zero which is read in
entry.S and put in $19.
-aneesh
On Thu, 2003-09-18 at 11:12, Kumar, Aneesh wrote:
> On Thu, 2003-09-18 at 01:30, Marcelo Tosatti wrote:
> > On Wed, 10 Sep 2003, Bjorn Helgaas wrote:
> >
> > > Here's a 2.4 backport of this change to 2.5:
> > >
> > >
> >
> http://linux.bkbits.net:8080/linux-2.5/[email protected]?nav=index.html
> > >
> > > Alpha, ppc, and sparc64 define force_successful_syscall_return() in
> > 2.5,
> > > but since it's not obvious to me how to do it correctly in 2.4, I
> left
> > > them unchanged.
> >
> > Whats the reasoning behing this patch?
>
> IIRC those changes were added to 2.5 by David. Architecture like Ia64
> and Alpha support error return via a different register set ( $19 for
> Alpha ). But syscalls like ptrace can have negative return value for
> successful returns. So in that particular case $19 is forced to be zero
> to indicate it is a successful return. IIUC
> force_successful_syscall_return is a wrapper around doing that. On
> alpha actually r0 in the stack (regs.r0 ) is made zero which is read in
> entry.S and put in $19.
For IA64 I guess it is r10 and regs.r8. May be other can correct me if i
am wrong.
-aneesh