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Message-Id: <20230703111541.496900-1-falcon@tinylab.org>
Date: Mon, 3 Jul 2023 19:15:41 +0800
From: Zhangjin Wu <falcon@...ylab.org>
To: w@....eu
Cc: arnd@...db.de, david.laight@...lab.com, falcon@...ylab.org,
linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
linux-riscv@...ts.infradead.org, thomas@...ch.de
Subject: Re: [PATCH v5 10/14] tools/nolibc: __sysret: support syscalls who return a pointer
Hi, Willy
> On Mon, Jul 03, 2023 at 04:36:51PM +0800, Zhangjin Wu wrote:
> > > Syscalls that return pointer use that -MAX_ERRNO range to encode errors
> > > (such as mmap()). I just do not know if there is a convention saying that
> > > other ones also restrict themselves to that range or not. If you find
> > > some info which guarantees that it's the case for all of them, then by
> > > all means let's proceed like this, but in this case it should be mentioned
> > > in the comment why we think it's valid to do this. For now it's presented
> > > as an opportunity only.
> >
> > Currently, I only found a prove-in-use case in musl:
> >
> > https://elixir.bootlin.com/musl/latest/source/src/internal/syscall_ret.c:
> >
> > #include <errno.h>
> > #include "syscall.h"
> >
> > long __syscall_ret(unsigned long r)
> > {
> > if (r > -4096UL) {
> > errno = -r;
> > return -1;
> > }
> > return r;
> > }
> >
> > Our new implementation (based on the one used by mmap()) is almostly the same
> > as musl. Not sure if this is enough. I have tried to 'git blame' on
> > __syscall_ret() of musl to find some clue, but failed, because the function has
> > been added before importing into its git repo.
>
> OK, we already used the glibc-saved registers in the past to determine
> the official list of clobbered registers (and the ABI spec was even
> updated based on this). Here, musl is sufficiently deployed to consider
> this as valid. You can simply go that route and mention in the commit
> message that while you found no official reference stating that this is
> valid for int/long returns, you found at least one other implementation
> relying on this (i.e. if the kernel ever changes it will cause breakage).
>
ok.
> > > Also, the rest of the commit message regarding uintptr_t (which we don't
> > > use), bit values and modular arithmetics is extremely confusing and not
> > > needed at all. What matters is only to know if we need to consider only
> > > values -MAX_ERRNO..-1 as error or all negative ones. If so, then it's
> > > obvious that ret >= (unsigned long)-MAX_ERRNO catches them all, as the
> > > current mmap() function already does with -4095UL.
> > >
> >
> > Yes, will clean up the commit message, but at first, let's continue get
> > more information about which one is ok:
> >
> > - -MAX_ERRNO..-1 as error, for sys_mmap (we know in nolibc) currently
> >
> > - all negative ones, for others currently
>
> You can double-check in glibc for example, but I'm starting to guess
> you'll find the same test as above, i.e. errors are exclusively >-4096,
> regardless of the expected return type.
>
Your guest is definitely true ;-)
Glibc has the same logic in its INLINE_SYSCALL() macro:
https://elixir.bootlin.com/glibc/latest/source/sysdeps/unix/sysv/linux/sysdep.h
#undef INTERNAL_SYSCALL_ERROR_P
#define INTERNAL_SYSCALL_ERROR_P(val) \
((unsigned long int) (val) > -4096UL)
#ifndef SYSCALL_ERROR_LABEL
# define SYSCALL_ERROR_LABEL(sc_err) \
({ \
__set_errno (sc_err); \
-1L; \
})
#endif
/* Define a macro which expands into the inline wrapper code for a system
call. It sets the errno and returns -1 on a failure, or the syscall
return value otherwise. */
#undef INLINE_SYSCALL
#define INLINE_SYSCALL(name, nr, args...) \
({ \
long int sc_ret = INTERNAL_SYSCALL (name, nr, args); \
__glibc_unlikely (INTERNAL_SYSCALL_ERROR_P (sc_ret)) \
? SYSCALL_ERROR_LABEL (INTERNAL_SYSCALL_ERRNO (sc_ret)) \
: sc_ret; \
})
Nothing differs.
But 'git blame' has no clue to any 'spec' or 'standard' either.
- fcb78a55058fd, linux: Consolidate INLINE_SYSCALL
Moved all of the arch specific INTERNAL_SYSCALL_ERROR_P() to common
header
- 369b849f1a382, sysdeps/unix/sysv/linux/s390/s390-32/sysdep.h (INTERNAL_SYSCALL,...
Firstly defined this macro: INTERNAL_SYSCALL_ERROR_P()
$ git show 369b849f1a3 | grep "define.*INTERNAL_SYSCALL_ERROR_P"
+#define INTERNAL_SYSCALL_ERROR_P(val) ((unsigned int) (val) >= 0xfffff001u)
+#define INTERNAL_SYSCALL_ERROR_P(val) ((unsigned int) (val) >= 0xfffff001u)
+#define INTERNAL_SYSCALL_ERROR_P(val) ((unsigned long) (val) >= -515L)
+#define INTERNAL_SYSCALL_ERROR_P(val) ((unsigned long) (val) >= -4095L)
Willy, I plan to further use something like, is it ok for you?
tools/include/nolibc/errno.h:
-#define MAX_ERRNO 4095
+#define MAX_ERRNO 4095UL
tools/include/nolibc/sys.h:
/* Syscall return helper for library routines
* set errno as -ret when ret in [-MAX_ERRNO, -1]
*
* Note, No official reference states the errno range
* here aligns with musl (src/internal/syscall_ret.c)
* and glibc (sysdeps/unix/sysv/linux/sysdep.h)
*/
static __inline__ __attribute__((unused, always_inline))
long __sysret(unsigned long ret)
{
if (ret >= -MAX_ERRNO) {
SET_ERRNO(-(long)ret);
return -1;
}
return ret;
}
Or we also directly use 4096UL here.
static __inline__ __attribute__((unused, always_inline))
long __sysret(unsigned long ret)
{
if (ret > -4096UL) {
SET_ERRNO(-(long)ret);
return -1;
}
return ret;
}
Best regards,
Zhangjin
> Thanks!
> Willy
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