Linux IO Multiplexing for Samba bug
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This document discusses different methods of I/O multiplexing including select, poll, and asynchronous I/O. It specifically describes issues with negative timeouts in the Samba smbd process when using poll and proposes solutions such as modifying the timeout to be 1 if negative or leveraging the default parent loop timeout of 60 seconds.
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---------------------------------------------------
Speaker: Victor Chong
Date: February 11, 2015
---------------------------------------------------
★ Session Summary ★
Explains the building blocks involved in Security including TrustZone, OP-TEE, Trusted Firmware etc. Goes into detail on how Secure Boot Works.. and Why. Explains how a simple secure Trusted Application interacts with OP-TEE and works. Brief overview on how to port OP-TEE to an ARM platform. Opens discussions for Potential Challenges and Hardware limitations and how they can be overcome.
--------------------------------------------------
★ Resources ★
Pathable: https://hkg15.pathable.com/meetings/250816
Video: https://www.youtube.com/watch?v=Fksx4-bpHRY
Etherpad: http://pad.linaro.org/p/hkg15-311
---------------------------------------------------
★ Event Details ★
Linaro Connect Hong Kong 2015 - #HKG15
February 9-13th, 2015
Regal Airport Hotel Hong Kong Airport
---------------------------------------------------
http://www.linaro.org
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Open Enea Linux workshop at the Embedded Conference Scandinavia 2014
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#Prerequisites:
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Linux IO Multiplexing for Samba bug
- 2. Problem • Samba smbd will crash when timeout is negative • Analysis FreeBSD poll and select…
- 3. Descriptive Name File Number Description StandardIn 0 Inputfromthe keyboard StandardOut 1 Outputtothe console StandardError 2 Error outputtothe console Process File descriptor
- 4. Five IO models blocking I/O: blocked all the way non-blocking I/O: if no data in buffer, immediate returns EWOULDBLOCK I/O multiplexing (select and poll): blocked separately in wait and copy signal driven I/O (SIGIO): non-blocked in wait but blocked in copy (signaled when I/O can be initiated) asynchronous I/O (aio_): non-blocked all the way (signaled when I/O is complete) Kernel’s buffer IO Device User process Wait for data copy date (system call)
- 6. Select 1.Is the device readyfor reading? (writing 2.Kernel check the device buffer’s limits ->READY for reading(writing 3.Select return IO Device Kernel’s buffer User process INTPUT OUTPUT 4.Copy data by system call(READ WRITE no yes 4. next Select()
- 7. Select • #include <sys/select.h> • int select( int nfds, fd_set *readset, fd_set *writeset, fd_set *exceptset, struct timeval *timeout) • struct timeval { /* Epoch 1970 1/1 00:00 */ long tv_sec; /* seconds*/ long tv_usec; /* microseconds*/ } Timeout NULL blocking infinite time timout->tv-sec=10 wait ten seconds timout->tv-sec=10 =0 search once and return nfds Search range 0 0 0 0 0 0 0 0 fd0 fd1023 0 0 0 0 0 0 0 0 0 fd0 fd1023 0 0 0 0 0 0 0 0 0 fd0 fd1023 0 readset writeset execptset FILE descriptors
- 8. 0 0 0 0 0 0 0 0 fd0 fd1023 FD_ZERO(fd_set *fdset) initial 0 0 0 0 0 1 0 0 0 fd0 fd1023 FD_SET(int fd,fd_set *fdset) loop 0 0 0 0 0 0 0 0 0 fd0 fd1023 FD_CLR(int fd,fd_set *fdset) Don’t use the device 0 0 0 0 0 0 0 0 0 fd0 fd1023 FD_ISSET(int fd,fd_set *fdset) Check ready? ->READ or WRITE 0 Check ready?
- 9. POLL poll -- synchronousI/Omultiplexing int poll (structpollfd fds[], nfds_t nfds, int timeout); 1.沒有最大文件描述符數量的限制。 2.Overhead is liners with number of file descriptions
- 10. POLL
- 11. Samba poll problem • sys_poll don’t handle timeout < 0 • Solve issue – 1.superviosr control • But…..Supervisoralso have negativetimeout problem – 2.moodify timeout=1 , if timeout<0 • 2014-2016 negative • 5- INT_MAX overflow – 3.smd_parent_loopdefaultis 60s • default sys_polltimeout range – /*tv_usec < 10^6 */ – tv_usec and tv_sec is positve