[ECOS] file stat mode bits and compatibility

[ECOS] file stat mode bits and compatibility

[Rutger Hofman]

I wanted to share a YAFFS file system between my eCos application and my 
u-boot bootloader. Well, since the NAND flash implementations eCos has 
YAFFS, and u-boot has it too. I ran into a problem when sharing a YAFFS 
file system. The single incompatibility issue is the value of file stat 
mode bits. E.g., u-boot has S_IRUSR = S_IREAD = (1<<8) but eCos has 
(1<<16). Now, this incompatibility will not only bite me, but anyone who 
wants to share a YAFFS file system between RedBoot and Linux or whatever 
combination of systems. There seems to be no hook in YAFFS to disable 
the permissions altogether.

POSIX does not appear to prescribe values for these bits 
(http://www.opengroup.org/onlinepubs/000095399/basedefs/sys/stat.h.html), 
so an implementation appears free to choose them. A short investigation 
showed that Linux, Solaris, and M$ Visual C++ all have chosen the same 
values as u-boot, which makes eCos the odd one out.

Just changing the eCos mode bit definitions will render existing file 
systems unusable. So, this proposal: introduce a .cdl option 
CYGBLD_ISO_STAT_MODE_COMPATIBILITY in isoinfra.cdl which, when enabled, 
has the mode bit definitions like Linux, u-boot, and the rest; and when 
disabled (as by default?), has the old situation.

Enabling this stat mode compatibility made the YAFFS file system fully 
accessible between u-boot and eCos.

What is the opinion of the list?

If people agree, shall I submit a patch to the ecos-patches list?

Rutger Hofman
VU Amsterdam

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Re: [ECOS] file stat mode bits and compatibility

[Ross Younger]

Rutger Hofman wrote:
> I wanted to share a YAFFS file system between my eCos application and my
> u-boot bootloader. [...]
> The single incompatibility issue is the value of file stat mode bits.

This is something I wondered about (with eCos-Linux filesystem compatibility
in mind), but haven't had the time to give much thought to. Your proposal
seems reasonable to me; I agree that it ought to generally default off so as
not to break the existing behaviour. Perhaps, if we foresaw certain boards
as likely to need this option, we might require it in their platform HAL?


Ross

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Embedded Software Engineer, eCosCentric Limited.
Barnwell House, Barnwell Drive, Cambridge CB5 8UU, UK.
Registered in England no. 4422071.                  www.ecoscentric.com

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