should be good change too this driver to enable the RS-485 ________________________________ De: kr....@kerogit.eu <kr....@kerogit.eu> Enviado: segunda-feira, 2 de junho de 2025 16:58 Para: dev@nuttx.apache.org <dev@nuttx.apache.org> Assunto: Re: Sv: RFC: fix race conditions in drivers/serial/serial.c
Hello, I noticed a review from Allan C. Assis about a spelling error. Updated patches are attached and also available in repository in uart_fixes_rfc2 branch. (Sorry about the extra work, I was even trying to check the word using web search... but I didn't spot that the search autocorrected it so my conclusion was that the spellchecker I also used is just tripping on the plural.) Best regards On 2025-06-02 09:39, alin.jerpe...@sony.com wrote: > Hi KR, > > thank for submitting the patches > > They have been uploaded to mainline and are under review > https://github.com/apache/nuttx/pull/16466 > > > Best regards > Alin > ________________________________ > Från: kr....@kerogit.eu <kr....@kerogit.eu> > Skickat: den 1 juni 2025 22:42 > Till: dev@nuttx.apache.org <dev@nuttx.apache.org> > Ämne: RFC: fix race conditions in drivers/serial/serial.c > > While going through the code in drivers/serial/serial. c, I noticed > this comment: The head and tail pointers are 16-bit values. The only > time that the following could be unsafe is if the CPU made two > non-atomic 8-bit accesses to obtain the 16-bit > > > While going through the code in drivers/serial/serial.c, I noticed this > comment: > > The head and tail pointers are 16-bit values. The only time that > the following could be unsafe is if the CPU made two non-atomic > 8-bit accesses to obtain the 16-bit head index. > > This is what happens for (at least) AVR architecture. These are 8bit > microcontrollers and as such, they will fetch the 16-bit value in two > 8-bit load instructions; interrupt routine may execute between those > and > change the value being read. This will result in corrupted read (one > byte of the value will be from pre-interrupt state and the other from > post-interrupt state.) > > This patch introduces CONFIG_ARCH_LD_16BIT_NOT_ATOMIC configuration > option, which is automatically selected for architectures known to be > unable to load 16bit values in one instruction. (It is currently only > set for AVR. I presume it might be also useful for Z80 but I do not > have > any experience with that architecture so I did no changes there.) When > this configuration option is set, reads of recv.head and xmit.tail are > enclosed with enter_critical_section and leave_critical_section calls > to > prevent interrupt handlers from running, if needed. Not all reads need > to be protected this way - some are already in existing critical > sections and some happen with the UART-specific interrupt disabled. > > If the configuration option is not set, the code simply loads the value > into a local variable. Subsequent direct uses of the unprotected > volatile variable are replaced with the local variables for both cases. > > There is also a related change that only applies when > CONFIG_SERIAL_IFLOWCONTROL_WATERMARKS is set. Aside from the protection > selected by CONFIG_ARCH_LD_16BIT_NOT_ATOMIC, this patch also fixes > calculation of the nbuffered value. This calculation is not running > with > interrupts disabled and value of rxbuf->head can change between the > condition and actual computation. Even if the load itself is atomic, > this leads to TOCTOU error. > > Impact: this change should not impact architectures that do not benefit > from this change at all unless CONFIG_SERIAL_IFLOWCONTROL is set. If > CONFIG_SERIAL_IFLOWCONTROL is set, the only change that remains in > effect is the fix of the TOCTOU error. > > Testing: Patch was tested with rv-virt:nsh - disassembly of functions > from drivers/serial/serial.c was compared and did not change after the > patch was applied - with one exception. The exception was an address of > a global variable, I assume it was caused by change of g_version length > (which notes that the tree is dirty) and is therefore inconsequential. > CONFIG_SERIAL_IFLOWCONTROL was not set in this test. > > Configuration with CONFIG_SERIAL_IFLOWCONTROL set was tested by > building > it for AVR DA/DB. Configuration with CONFIG_SERIAL_IFLOWCONTROL unset > was tested by custom echo application running on AVR128DA28 for a few > hours. > > I also ran CI test: 1044 passed, 10 skipped, 14 deselected, 3 warnings > in 2463.23s (0:41:03) > > > On a related note, it seems to me that there may be a bug in echo > handling in uart_readv. If dev->tc_lflag & ECHO is true, then > uart_putxmitchar is called. This function adds the received character > to > transmit buffer and increments xmit.head value. However, > uart_putxmitchar is also called from uart_writev; this function states > "Only one user can access dev->xmit.head at a time" and takes the > dev->xmit.lock. As far as I can see, uart_readv does not take the lock > and may interfere with uart_writev. Evaluating and solving this > properly > is currently above my skillset though. (If this is even considered > serious enough to warrant a fix.) > > > The patch series also fixes a typo in drivers/serial/serial.c > > > Both patches are attached to this message and also available in a git > repository nuttx.git at git.kerogit.eu accessible through HTTP/S. > (Trying to prevent bot traffic by not posting the URL in > machine-readable form.) The relevant branch is called uart_fixes_rfc1. > If the patches are acceptable, I would like to ask someone with GitHub > account to open a PR (I don't have a GH account.) Any comments or > suggestions are welcome.
