Bug 5540

Summary:	Parse error in ash when redirecting value to sysfs
Product:	Busybox	Reporter:	Henri Bragge <henri.bragge>
Component:	Standard Compliance	Assignee:	unassigned
Status:	RESOLVED INVALID
Severity:	minor	CC:	busybox-cvs
Priority:	P5
Version:	unspecified
Target Milestone:	---
Hardware:	All
OS:	All
Host:		Target:
Build:
Attachments:	echo 128> ... echo 128 > ...

Description Henri Bragge 2012-09-19 07:25:10 UTC

When echoing a value and redirecting it to a file under sysfs, ash just prints an empty line but does not store the value. This only happens when ">" character is typed right after the command.

For example, this works:
# echo 250 > /sys/class/backlight/pwm-backlight.0/brightness
# cat /sys/class/backlight/pwm-backlight.0/brightness
250

But this does not:
# echo 128> /sys/class/backlight/pwm-backlight.0/brightness

# cat /sys/class/backlight/pwm-backlight.0/brightness
250

Redirecting in similar way to a normal file works:
# echo hello> /tmp/world
# cat /tmp/world
hello

Comment 1 Henri Bragge 2012-09-19 07:25:48 UTC

Tested with busybox 1.19.4 and 1.20.2

Comment 2 Denys Vlasenko 2012-09-19 09:34:45 UTC

Please strace it:

strace -oLOG -s99 ash -c 'echo 128> /sys/class/backlight/pwm-backlight.0/brightness'

and post the LOG file

Comment 3 Henri Bragge 2012-09-19 09:56:45 UTC

Created attachment 4538 [details]
echo 128> ...

Comment 4 Henri Bragge 2012-09-19 10:00:50 UTC

Created attachment 4544 [details]
echo 128 > ...

In LOG-error, there is something interesting in line 48:
fcntl64(128, F_DUPFD, 10)               = -1 EBADF (Bad file descriptor)
dup2(3, 128)                            = 128
close(3)                                = 0
wait4(-1, 0x7e8fa6dc, WNOHANG, NULL)    = -1 ECHILD (No child processes)
write(1, "\n", 1)                       = 1
close(128)                              = 0

In LOG-ok, this looks like:
fcntl64(1, F_DUPFD, 10)                 = 10
dup2(3, 1)                              = 1
close(3)                                = 0
wait4(-1, 0x7ecc66dc, WNOHANG, NULL)    = -1 ECHILD (No child processes)
write(1, "128\n", 4)                    = 4
dup2(10, 1)                             = 1
close(10)                               = 0

Comment 5 Henri Bragge 2012-09-19 10:20:34 UTC

Small correction, though you probably noticed it already. Problem actually occurs similarly in normal files and sysfs. Instead, there is something special in the way ash handles numbers that are not separated from ">" character with a whitespace:

Both of these do not work as expected:
# echo 66> /sys/class/backlight/pwm-backlight.0/brightness

# echo 66>/tmp/world

# cat /tmp/world
#

Comment 6 Henri Bragge 2012-09-19 10:32:22 UTC

I tested this again with bash on my PC and it seems to behave exactly the same (dealing numbers as file handles?). Nothing strange in it after all.

Maybe should have given it a bit more thought before reporting :)