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Date: Sat, 29 Aug 2020 00:13:28 -0700 From: Joe Perches <joe@...ches.com> To: Denis Efremov <efremov@...ux.com>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, "Rafael J. Wysocki" <rafael@...nel.org> Cc: Kees Cook <keescook@...omium.org>, "Gustavo A . R . Silva" <gustavoars@...nel.org>, Julia Lawall <julia.lawall@...ia.fr>, Alex Dewar <alex.dewar90@...il.com>, linux-kernel@...r.kernel.org Subject: Re: [PATCH] sysfs: Add sysfs_emit to replace sprintf to PAGE_SIZE buffers. On Sat, 2020-08-29 at 09:59 +0300, Denis Efremov wrote: > Hi, > > On 8/29/20 1:52 AM, Joe Perches wrote: > > sprintf does not know the PAGE_SIZE maximum of the temporary buffer > > used for outputting sysfs content requests and it's possible to > > overrun the buffer length. > > > > Add a generic sysfs_emit mechanism that knows that the size of the > > temporary buffer and ensures that no overrun is done. > > > > Signed-off-by: Joe Perches <joe@...ches.com> > > --- rK > > It could be a good idea to update the docs to, i.e.: > https://www.kernel.org/doc/html/latest/filesystems/sysfs.html Yes, thanks. I have the below already, but Greg makes a sensible point about the generic use of sysfs_emit for single values which is ~95% of the actual uses, so likely there will be two functions. Given the multiple thousand instances, using 2 functions would be smaller overall object code as well. Perhaps: sysfs_emit (for single value output) sysfs_emit_at (or sysfs_emit_pos ? or some better name?) int sysfs_emit(char *buf, const char *fmt, ...) int sysfs_emit_at(char *buf, int pos, const char *fmt, ...) or maybe use int sysfs_emit_pos(char *buf, char *pos, const char *fmt, ...) The multiple use emit_at with int as the 2nd parameter would make the direct return easier than the char * which needs a subtraction. six of this/half dozen of that... cheers... --- Anyway, this will need updating, likely with better examples. diff --git a/Documentation/filesystems/sysfs.rst b/Documentation/filesystems/sysfs.rst index ab0f7795792b..13c7a86fa6c8 100644 --- a/Documentation/filesystems/sysfs.rst +++ b/Documentation/filesystems/sysfs.rst @@ -242,12 +242,9 @@ Other notes: is 4096. - show() methods should return the number of bytes printed into the - buffer. This is the return value of scnprintf(). + buffer. This is the return value of sysfs_emit(). -- show() must not use snprintf() when formatting the value to be - returned to user space. If you can guarantee that an overflow - will never happen you can use sprintf() otherwise you must use - scnprintf(). +- show() methods should only use sysfs_emit to format output.
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