And, importantly, it helps the kernel developers to keep overall kernel. This is wasteful, imo, es the unix and linux forums. When i use the memcpy, the maximum payload for a tlp is 8 bytes on 64 bits architectures. A description of the project, information about reporting bugs, and the latest version of this page, can be found at s. If the source and destination overlap, this function does not ensure that the original source bytes in the overlapping region are copied before being overwritten. This helper moves the initial part of head, up to and including entry, from head to list. The memcpy function copies count bytes of src to dest.
Seems like visual studio implements a safe memcpy along with gcc 4. Afaik glibc does get better results than the kernel approach, but theyve also introduced bugs3 that way and the code is very complex by comparison. Both dont check the terminating null character in the string. And so began a slow conversion of the many bcopy calls in the kernel to memcpy. Hardware acceleration for memory to memory copies uc berkeley. It is usually more efficient than strcpy, which must scan the data it copies or memmove, which must take precautions to handle overlapping inputs. If the source and destination objects overlap, the behavior of memcpy is undefined. How to use memcpy in kernel driver kristof provost kristof at sigsegv. Efficient data copy from pcie device to ram in kernel. There is a man page in linux for the memove which clearly specifies that memove will copy data in temporary variable. This does not mean that the actual implementation uses any buffer, but probably does some pointer arithmetic. How to use memcpy in kernel driver messages sorted by.
If the source and destination overlap, the behavior of memcpy is undefined. Im writing a device driver in linux for a pcie device. The c standard specifies two functions for copying memory regions, memcpy and memmove. So a lot of code that runs fine on linux would blow up on solaris. The chances are that memcpy is faster less expensive because its more frequently used, and there are plenty of optimized memcpy implementations out there, whilst memmove is less frequently used, so the chances are that its slightly less optimized in the first place, and has added complexity, whcih can cause it to be slower in itself. The memcpy is useful in forwarding copy but memmove is useful in case of overlapping scenario. Mempcpy3 linux programmers manual mempcpy3 name top mempcpy, wmempcpy copy memory area. Instead, the code resulting for these functions is a call memcpy, a function that doesnt exist in the kernel. Id say its more likely that such a bug would be in the implementation of mmap support at the driver level, not with memcpy itself. When writing drivers, you cannot in general use routines which are from the c library. The result of memmove is defined as if the src was copied into a buffer and then buffer copied into dst. The linux kernel api the linux kernel documentation. Library linux docs linux man pages page load time toys world sunlight moon phase trace explorer. Flash users, meanwhile, are still waiting for adobe to change a memcpy call to memmove.
Unlike memcpy, memmove copes with overlapping areas. The linux kernel provides more basic utility functions. Description top the memmove function copies n bytes from memory area src to memory area dest. Because the kernel can write to any address it wants, if you just use a userspace address you got and use memcpy, an attacker could write to another processs pages, which is a huge. One can also compile on an older machine, or compile a private version of the toolchain. Browse other questions tagged c linux kernel linux device driver memcpy or ask your own question. Because so many buffer overruns, and thus potential security exploits, have been traced to improper usage of memcpy, this function is listed among the banned functions by the security development lifecycle sdl. In the kernel, the optimized x86 memcpy we use is actually a memmove, because while performance is really important, so is repeatability and avoiding surprises strictly speaking, we have two. This requires inspecting the function arguments to make sure. Experimental benchmark and test toolkit for optimized arm memcpymemset functions in the linux kernel hglmtest armkernelmemcpy.
This device driver performs several read and write to test the throughput. Memmove on the laptop runs slower than memcpy but oddly enough runs at the same speed as the memmove on the server. Description top the memccpy function copies no more than n bytes from memory area src to memory area dest, stopping when the character c is found. Using any callmechanism on a hardware generated interrupt, the interrupt handler isr wakes the sleeping userthread. Description the memmove function copies n bytes from memory area src to memory area dest.
The c program to implement memcpy and memmove is developed in linux ubuntu operating system and compiled with gcc compiler. Memory memory copies are used especially heavily in kernel io drivers such as disk and network device. The linux kernels goal of being very portable requires this kind of abstraction. Click hereyou are currently subscribed to ntdev as. Before attacking any of these, we suggest sending a few patches in advance to see if you are doing something wrong, or if someone else is already doing same work read related threads in the mailing list archive.
Linus was butting in because a kernel sound driver regression was also. The important difference is that it is undefined behavior to call memcpy with overlapping regions. This copy routine is memcpycompatible, but on some architectures will use cachebypassing loads to avoid bringing the source data into the cache. In my opinion the only way to get a payload of 16 bytes is to use the sse instruction set. Contribute to torvaldslinux development by creating an account on github.
A handwritten memmove is faster in microbenchmarks, but the icache effects may make the overall performance difference smaller or even negative. Writing the versions of these functions from gives a clear picture. Sure, in a perfect world, a careful uberprogrammer can perform those checks herself but i have still found way too many mistakes, even offbyone errors, even from some of the greatest programmers out there, to think that a mistake in this. The memmove function is slower in comparison to memcpy because in memmove extra temporary array is used to copy n characters from the source and after that, it uses to copy the stored characters to the destination memory. As already pointed out in other answers, memmove is more sophisticated than memcpy such that it accounts for memory overlaps. Description top the memcpy function copies n bytes from memory area src to memory area dest. Undefined references to memcpy when compiling linux kernel. Efficient data copy from pcie device to ram in kernel hello, i have got an fpga card 4 pcie v1.
Its fun to benchmark memmove and memcpy on a box to see if memcpy has more optimizations or not. The use of temp buffer in memmove is due to the reason that, in memmove, the memory areas may overlap. Contribute to hucekexine libvaapi development by creating an account on github. A few notes about memcpy vs memmove and some related items as well. Should open method in linux device driver return a file descriptor. Every implementation of memcpy and memmove that ive seen has been. Hi all, is it possible to do the following in linux kernel 2. Linux device driver linux kernel internals networking uncategorized. Description the memcpy function copies n bytes from memory area src to memory area dest. Based on some experimentation i have tried using memmove instead of memcpy in my test case and have found a 2x improvement on the server. If the user provides a cleverly crafted pointer, memcpy will happily copy kernel data. I hate to ask this sort of thing, but youre not taking the address of the variable storing the mmap pointer or using the size of that pointer in memcpy, are you. It zeros the memory and, as such, will touch every page allocated, forcing the kernel to actually create a physical page for it.