From: "Andreas Dröscher" <musl@bugfree.ch>
To: musl@lists.openwall.com
Subject: Re: [musl] mips32 little endian -ENOSYS is not -(-ENOSYS)
Date: Wed, 11 Mar 2020 02:19:31 +0100 [thread overview]
Message-ID: <bb74d46a-8f2e-08b8-f19e-dc6bb4ad97a3@droescher.ch> (raw)
In-Reply-To: <20200311005553.GE11469@brightrain.aerifal.cx>
Am 11.03.20 um 01:55 schrieb Rich Felker:
> On Tue, Mar 10, 2020 at 10:10:15PM +0100, Andreas Dröscher wrote:
>> Hi
>>
>> I'm building a new toolchain for a very old hardware with a very old
>> Linux Kernel (2.6.20). The CPU is a Alchemy (now AMD) AU1100
>> (production was discontinued).
>>
>> Obviously the Kernel lacks a lot of the modern system calls. I
>> however expect the general system call interface to be consistent.
>> Moreover, musl has fallbacks for many system-calls in place, kudos!
>> However, the fallback is never triggered. I will present the issue
>> on one example (epoll):
>>
>> excerpt from src/linux/epoll.c:
>> int epoll_create1(int flags)
>> {
>> int r = __syscall(SYS_epoll_create1, flags);
>> #ifdef SYS_epoll_create
>> if (r==-ENOSYS && !flags) r = __syscall(SYS_epoll_create, 1);
>> #endif
>> return __syscall_ret(r);
>> }
>>
>> If r is -89 (negative ENOSYS) the fallback is triggered else the
>> result is returned as it is. However, in my case __syscall returnes
>> 89 (positive ENOSYS).
>> I've tracked the return into the kernel and there the negative value
>> is returned. The Kernel additionally sets r7 to 1.
>>
>> excerpt from arch/mips/syscall_arch.h:
>> static inline long __syscall1(long n, long a)
>> {
>> register long r4 __asm__("$4") = a;
>> register long r7 __asm__("$7");
>> register long r2 __asm__("$2") = n;
>> __asm__ __volatile__ (
>> "syscall"
>> : "+r"(r2), "=r"(r7)
>> : "r"(r4)
>> : SYSCALL_CLOBBERLIST, "$8", "$9", "$10");
>> return r7 ? -r2 : r2;
>> }
>>
>> I assume the "bug" is triggered by __syscall1 If r7 is set it will
>> change the sign of r2. I can patch that by replacing:
>> return r7 ? -r2 : r2;
>> with
>> return (r7 && r2 > 0) ? -r2 : r2;
>>
>> However I've no idea if I'm triggering any side effects or if I
>> selected the wrong implementation for my architecture.
>
> It sounds like what you're saying is that the ENOSYS codepath for
> mips, at least on your old kernel, is not setting the error flag in r7
> and returning ENOSYS in r2, but is instead returning -ENOSYS already
> (and not clear whether it's setting r7 at all or just leaving a stale
> value there).
>
> Can anyone else confirm this, or point to kernel history that might
> suggest it's a real bug? Your workaround looks like it should at least
> be *safe* to do, and probably the right thing if this was/is a real
> kernel bug in the official kernel rather than something some vendor
> broke in their fork.
>
> Rich
>
Sorry for not including that excerpt in the first place:
illegal_syscall:
li v0, -ENOSYS # error
sw v0, PT_R2(sp)
li t0, 1 # set error flag
sw t0, PT_R7(sp)
j o32_syscall_exit
END(handle_sys)
Source:
https://github.com/torvalds/linux/blob/62d0cfcb27cf755cebdc93ca95dabc83608007cd/arch/mips/kernel/scall32-o32.S#L186
next prev parent reply other threads:[~2020-03-11 1:19 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-10 21:10 Andreas Dröscher
2020-03-11 0:55 ` Rich Felker
2020-03-11 1:19 ` Andreas Dröscher [this message]
2020-03-11 1:40 ` Rich Felker
2020-03-11 2:08 ` Andreas Dröscher
2020-03-11 2:18 ` Rich Felker
2020-03-11 21:08 ` Andreas Dröscher
2020-03-11 21:35 ` Rich Felker
2020-03-11 23:08 ` Rich Felker
2020-03-20 16:34 ` Rich Felker
2020-03-20 22:18 ` Andreas Dröscher
2020-03-20 22:33 ` Rich Felker
2020-03-29 21:46 ` Andreas Dröscher
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=bb74d46a-8f2e-08b8-f19e-dc6bb4ad97a3@droescher.ch \
--to=musl@bugfree.ch \
--cc=musl@lists.openwall.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
Code repositories for project(s) associated with this public inbox
https://git.vuxu.org/mirror/musl/
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).