Re: kern/53261: kernel crash during test run

[Kamil Rytarowski <n54%gmx.com@localhost>]

The following reply was made to PR kern/53261; it has been noted by GNATS.

From: Kamil Rytarowski <n54%gmx.com@localhost>
To: gnats-bugs%NetBSD.org@localhost
Cc: 
Subject: Re: kern/53261: kernel crash during test run
Date: Wed, 23 May 2018 22:52:18 +0200

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 From: Kamil Rytarowski <n54%gmx.com@localhost>
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 Subject: Re: kern/53261: kernel crash during test run
 References: <pr-kern-53261%gnats.netbsd.org@localhost>
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 On 23.05.2018 16:30, Martin Husemann wrote:
 > The following reply was made to PR kern/53261; it has been noted by GNA=
 TS.
 >=20
 > From: Martin Husemann <martin%duskware.de@localhost>
 > To: gnats-bugs%NetBSD.org@localhost
 > Cc:=20
 > Subject: Re: kern/53261: kernel crash during test run
 > Date: Wed, 23 May 2018 16:27:07 +0200
 >=20
 >  On Wed, May 23, 2018 at 02:05:00PM +0000, Kamil Rytarowski wrote:
 >  >  I need to repeat that I agree that it's not optimal, if it works it=
 's
 >  >  not portable.
 > =20
 >  Our documentation states that it is wrong. Allowing it to work by pure=
 
 >  luck on i386 just hides the problem, where otherwise tests would
 >  quickly show the culprit.
 > =20
 >  Why is it so difficult to call __syscall instead of syscall for the fe=
 w
 >  cases where it is need? I completly don't get your point.
 > =20
 >  Martin
 > =20
 >=20
 
 The ARM and EABI case is mentioned in the GNU/Linux man-page:
 
 http://man7.org/linux/man-pages/man2/syscall.2.html
 
        For example, on the ARM architecture Embedded ABI (EABI), a 64-bit=
 
        value (e.g., long long) must be aligned to an even register pair.
        Thus, using syscall() instead of the wrapper provided by glibc, th=
 e
        readahead() system call would be invoked as follows on the ARM
        architecture with the EABI in little endian mode:
 
            syscall(SYS_readahead, fd, 0,
                    (unsigned int) (offset & 0xFFFFFFFF),
                    (unsigned int) (offset >> 32),
                    count);
 
        Since the offset argument is 64 bits, and the first argument (fd) =
 is
        passed in r0, the caller must manually split and align the 64-bit
        value so that it is passed in the r2/r3 register pair.  That means=
 
        inserting a dummy value into r1 (the second argument of 0).  Care
        also must be taken so that the split follows endian conventions
        (according to the C ABI for the platform).
 
        Similar issues can occur on MIPS with the O32 ABI, on PowerPC with=
 
        the 32-bit ABI, and on Xtensa.
 
 
 My point is that there is nothing better for "indirect system call" and
 outlawing it or deteriorating its functionality does not help to its
 legitimate users... they are already aware about the potential problems
 with ABI or endian.
 
 
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