race-condition in ram_fs and fs_query?

Guido Witmond guido at witmond.nl
Mon May 20 13:27:16 CEST 2019


Hi Genodians,

I'm working on my HnH19-project to create a component listening on a 
shared filesystem to open each pdf file into a separate instance of 
mupdf. Goal is to tie the share to a linux-vm in vbox so firefox and 
thunderbird can have a simple 'cp' command to get pdfs rendered outside 
that vm.

I add a RAM-FS, FS-QUERY and REPORT-ROM to mupdf.run. The file gets put 
in the ram-fs, fs-query should pick that up and report it to the report-rom.

     <start name="pdf_share">
             <binary name="ram_fs"/>
             <resource name="RAM" quantum="128M"/>
             <provides> <service name="File_system"/> </provides>
             <config verbose="yes">
                     <default-policy root="/" writeable="yes" />
             </config>
     </start>

     <start name="report_new_pdf">
             <binary name="report_rom"/>
             <resource name="RAM" quantum="1M"/>
             <provides> <service name="Report"/> <service name="ROM"/> 
</provides>
             <config verbose="yes"/>
     </start>

     <start name="fs_query">
             <resource name="RAM" quantum="1M"/>
             <config verbose="yes">
                     <vfs> <fs writeable="yes"/> </vfs>
                     <query path="/" content="yes"/>
             </config>
     </start>

I change the test-scenario to copy a pdf into the share and start the 
pdf-viewer. The viewer picks up the pdf and renders it. So far so good.


The problem is that while the 'add'-test copies the file, it gets 
detected by fs_query but fs_query thinks it's gone:

[init -> test -> add] --- noux started ---
[init -> fs_query] Error: failed to watch '//foo.pdf'
[init -> fs_query] Warning: could not obtain content of nonexistent file 
foo.pdf

(The error comes from gems/vfs.h L556, the warning from fs_query/main.cc 
L80.)

Fs_quey does send a new report but does not show the file, it show an 
empty directory.

[init -> report_new_pdf] report 'fs_query -> listing'
[init -> report_new_pdf]   <listing>
[init -> report_new_pdf]        <dir path="/"/>
[init -> report_new_pdf]   </listing>

However, a ls -laR shows it's there:

[init -> test -> ls-after] /dest:
[init -> test -> ls-after] total 1
[init -> test -> ls-after] drwxr-xr-x 0 root 0    0 Jan  1 00:00 .
[init -> test -> ls-after] drwxr-xr-x 0 root 0    0 Jan  1 00:00 ..
[init -> test -> ls-after] -rwxrwxrwx 0 root 0 8192 Jan  1 00:00 foo.pdf


Now the race condition:

When I set the fs-query option to show the content to "no", it gives the 
same error: failed to watch '//foo.pdf' but it now it does send the file 
name to the report:

[init -> report_new_pdf]                <file name="foo.pdf"/>


So it seems that somehow that fs_query watch learns there is a file 
while the contents are not yet committed. Accessing that triggers a 
file-not-found error.

Did I discover a race condition or am I doing something wrong?


With regards, Guido.

PS. I added the complete run-file and logs for completeness. (log-1 
shows the contents-flag to no).
PPS. I'm testing with Qemu in a linux VM on my AMD nixos host.
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set build_components { app/pdf_view server/ram_fs app/fs_query app/sequence }

source ${genode_dir}/repos/base/run/platform_drv.inc

build $build_components

create_boot_directory

import_from_depot \
	[depot_user]/pkg/[drivers_interactive_pkg] \
	[depot_user]/pkg/motif_wm \
	[depot_user]/src/[base_src] \
	[depot_user]/src/init \
	[depot_user]/src/nitpicker \
	[depot_user]/src/coreutils \
	[depot_user]/src/bash \
	[depot_user]/src/init \
	[depot_user]/src/libc \
	[depot_user]/src/noux \
	[depot_user]/src/posix \
	[depot_user]/src/report_rom \
	[depot_user]/src/rom_to_file \
	[depot_user]/src/vfs \
	[depot_user]/src/vfs_import

set config {
<config verbose="yes">
	<parent-provides>
		<service name="ROM"/>
		<service name="IRQ"/>
		<service name="IO_MEM"/>
		<service name="IO_PORT"/>
		<service name="PD"/>
		<service name="RM"/>
		<service name="CPU"/>
		<service name="LOG"/>
	</parent-provides>
	<default-route>
		<service name="Nitpicker"> <child name="wm"/> </service>
		<any-service> <parent/> <any-child/> </any-service>
	</default-route>
	<default caps="100"/>

	<start name="timer">
		<resource name="RAM" quantum="1M"/>
		<provides><service name="Timer"/></provides>
	</start>

	<start name="drivers" caps="1000">
		<resource name="RAM" quantum="32M" constrain_phys="yes"/>
		<binary name="init"/>
		<route>
			<service name="ROM" label="config"> <parent label="drivers.config"/> </service>
			<service name="Timer"> <child name="timer"/> </service>
			<any-service> <parent/> </any-service>
		</route>
		<provides>
			<service name="Input"/> <service name="Framebuffer"/>
		</provides>
	</start>

	<start name="nitpicker">
		<resource name="RAM" quantum="4M"/>
		<provides><service name="Nitpicker"/></provides>
		<config focus="rom">
			<domain name="pointer" layer="1" content="client" label="no" origin="pointer" />
			<domain name="default" layer="2" content="client" label="no" hover="always"/>
			<policy label_prefix="pointer" domain="pointer"/>
			<default-policy domain="default"/>
		</config>
	</start>

	<start name="pointer">
		<resource name="RAM" quantum="1M"/>
		<route>
			<service name="Nitpicker"> <child name="nitpicker"/> </service>
			<any-service> <parent/> <any-child/> </any-service>
		</route>
	</start>

	<start name="wm" caps="1000">
		<resource name="RAM" quantum="32M"/>
		<binary name="init"/>
		<provides> <service name="Nitpicker"/> </provides>
		<route>
			<service name="ROM" label="config"> <parent label="wm.config"/> </service>
			<service name="Nitpicker"> <child name="nitpicker"/> </service>
			<any-service> <parent/> <any-child/> </any-service>
		</route>
	</start>


	<!-- pdf_share is the filesystem that Linux writes pdf files to. -->
	<start name="pdf_share">
		<binary name="ram_fs"/>
		<resource name="RAM" quantum="128M"/>
		<provides> <service name="File_system"/> </provides>
<!--
		<route>
			<any-service> <parent/> <any-child/> </any-service>
		</route>
-->
		<config verbose="yes">
			<default-policy root="/" writeable="yes" />
		</config>
	</start>

	<!-- report_new_pdf reports to the pdf-viewer when there is a new pdf -->
	<start name="report_new_pdf">
	        <binary name="report_rom"/>
		<resource name="RAM" quantum="1M"/>
		<provides> <service name="Report"/> <service name="ROM"/> </provides>
		<config verbose="yes"/>
	</start>

	<!-- fs_query monitors the pdf_share for new files and writes a report into report_new_pdf -->
	<start name="fs_query">
		<resource name="RAM" quantum="1M"/>
		<config verbose="yes">
			<!-- <vfs> <dir name="fs"> <fs writeable="yes"/> </dir> </vfs> -->
			<vfs> <fs writeable="yes"/> </vfs>
			<query path="/" content="no"/>
		</config>
<!--
		<route>
			<service name="File_system"> <child name="pdf_share" /> </service>
			<any-service> <parent/> <any-child/> </any-service>
		</route>
-->
	</start>


	<!-- TODO? rom_to_file picks up the new pdf from the report_new_pdf rom, makes it a file -->


	<start name="test" caps="700">
		<binary name="sequence"/>
		<resource name="RAM" quantum="64M"/>
		<config>
			<start name="sleep" caps="500">
				<binary name="noux"/>
				<config stdin="/dev/null" stdout="/dev/log" stderr="/dev/log">
					<fstab>
						<tar name="coreutils.tar" />
						<dir name="dev"> <log/> <null/> </dir>
					</fstab>
					<start name="/bin/sleep"> <arg value="2"/> </start>
				</config>
			</start>

			<start name="ls-before" caps="500">
				<binary name="noux"/>
				<config stdin="/dev/null" stdout="/dev/log" stderr="/dev/log">
					<fstab>
						<tar name="coreutils.tar" />
						<dir name="dest"> <fs writeable="yes"/> </dir>
						<dir name="dev"> <log/> <null/> </dir>
						<dir name="source"> <rom name="test.pdf" /> </dir>
					</fstab>
					<start name="/bin/ls">
					       <arg value="-laR" />
					       <arg value="/source" />
					       <arg value="/dest" />
					</start>
0				</config>
<!--
				<route>
					<!- - <service name="File_system"> <child name="pdf-view-fs" /> </service>- ->
					<service name="File_system"> <parent /> </service>
					<any-service> <parent/> <any-child/> </any-service>
				</route>
-->
			</start>

			<start name="add" caps="500">
				<binary name="noux"/>
				<config stdin="/dev/null" stdout="/dev/log" stderr="/dev/log">
					<fstab>
						<tar name="coreutils.tar" />
						<dir name="dest"> <fs writeable="yes"/> </dir>
						<dir name="dev"> <log/> <null/> </dir>
						<dir name="source"> <rom name="test.pdf" /> </dir>
					</fstab>

					<start name="/bin/cp">
						<arg value="/source/test.pdf" />
						<!-- <arg value="/dev/null" /> -->
						<arg value="/dest/foo.pdf" />
					</start>
				</config>
<!--
				<route>
					<!- - <service name="File_system"> <child name="pdf-view-fs" /> </service>- ->
					<service name="File_system"> <parent /> </service>
					<any-service> <parent/> <any-child/> </any-service>
				</route>
-->
			</start>

			<start name="ls-after" caps="500">
				<binary name="noux"/>
				<config stdin="/dev/null" stdout="/dev/log" stderr="/dev/log">
					<fstab>
						<tar name="coreutils.tar" />
						<dir name="dest"> <fs writeable="yes"/> </dir>
						<dir name="dev"> <log/> <null/> </dir>
						<dir name="source"> <rom name="test.pdf" /> </dir>
					</fstab>
					<start name="/bin/ls">
					       <arg value="-laR" />
					       <arg value="/source" />
					       <arg value="/dest" />
					</start>
				</config>
<!--
				<route>
					<!- - <service name="File_system"> <child name="pdf-view-fs" /> </service>- ->
					<service name="File_system"> <parent /> </service>
					<any-service> <parent/> <any-child/> </any-service>
				</route>
-->
			</start>

			<start name="pdf_view" caps="256">
			       <resource name="RAM" quantum="1G"/>
			       <config>
					<vfs>
						<dir name="dev"> <log/> </dir>
						<fs /> <!-- points to pdf_share -->
					</vfs>
					<libc stdout="/dev/log" stderr="/dev/log"/>
				</config>
<!--
				<route>
					<service name="Nitpicker">   <child name="nitpicker" /> </service>
					<service name="File_system"> <child name="pdf-view-fs" /> </service>
					<!- - /dev/log goes to any service LOG - ->
					<any-service> <parent/> <any-child/> </any-service>
				</route>
-->
			</start>

		</config>
<!--
		<route>
			<service name="File_system"> <child name="pdf_share" /> </service>
			<!- - <service name="File_system"> <parent /> </service> - ->
			<any-service> <any-child/> <parent/> </any-service>
		</route>
-->
	</start>


<!--
	<start name="rom_to_file">
		<resource name="RAM" quantum="2M"/>
		<config rom="dynamic_rom"/>
		<route>
			<service name="ROM" label="dynamic_rom"> <child name="dynamic_rom"/> </service>
			<service name="File_system" > <child name="vfs"/> </service>
			<any-service> <parent/> <any-child/> </any-service>
		</route>
	</start>
-->
	<!-- TODO: Feed the rom-to-file file to a fresh instance of pdf-view. -->

     <!-- start a dynamic sub-init here -->
<!--
	<start name="pdf-view-fs">
		<binary name="ram_fs"/>
		<resource name="RAM" quantum="256M"/>
		<provides> <service name="File_system"/> </provides>
		<route>
			<any-service> <parent/> <any-child/> </any-service>
		</route>
		<config>
			<default-policy root="/" writeable="yes" />
		</config>
	</start>

	<start name="pdf_view" caps="256">
		<resource name="RAM" quantum="1G"/>
		<config>
			<vfs>
				<dir name="dev"> <log/> </dir>
				<fs /> <!- - points to pdf_share - ->
			</vfs>
			<libc stdout="/dev/log" stderr="/dev/log"/>
		</config>
		<route>
			<service name="Nitpicker">   <child name="nitpicker" /> </service>
			<service name="File_system"> <child name="pdf-view-fs" /> </service>
			<!- - /dev/log goes to any service LOG - ->
			<any-service> <parent/> <any-child/> </any-service>
		</route>
	</start>
-->
	<!-- end sub-init here -->

<!--
	<start name="pdf_view" caps="256">
		<resource name="RAM" quantum="1G"/>
		<config>
			<vfs>
				<rom name="test.pdf" />
				<dir name="dev"> <log/> </dir>
			</vfs>
			<libc stdout="/dev/log" stderr="/dev/log"/>
		</config>
	</start>
-->

</config>
}

install_config $config

#
# Downloadotest PDF file
#
if {![file exist bin/test.pdf]} {
	set pdf_url "http://genode-labs.com/publications/genode-fpga-graphics-2009.pdf"
	catch { exec wget $pdf_url -O bin/test.pdf }
}

if {![file exist bin/test.pdf]} {
	puts stderr "Could not download test PDF from '$pdf_url'"
	exit 1
}

#
# Pin the nitpicker focus to the window manager by providing a static focus ROM
#
set fd [open [run_dir]/genode/focus w]
puts $fd "<focus label=\"wm -> focus\"/>"
close $fd

append boot_modules {
	libc.lib.so vfs.lib.so libm.lib.so
	openjpeg.lib.so freetype.lib.so libpng.lib.so zlib.lib.so jbig2dec.lib.so
	mupdf.lib.so jpeg.lib.so
	ram_fs
	fs_query sequence
	pdf_view
	test.pdf
}

build_boot_image $boot_modules

append qemu_args " -m 1536"

set env(SDL_VIDEO_X11_DGAMOUSE) 0

run_genode_until forever



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