Date: Fri, 29 Jul 2016 09:05:35 -0700 From: Conrad Meyer <cem@freebsd.org> To: Adam Starak <starak.adam@gmail.com> Cc: FreeBSD Hackers <freebsd-hackers@freebsd.org> Subject: Re: Modify user space from kernel. Message-ID: <CAG6CVpWdZMYqKLzz1H70Hq_SXF1eeOcEy0%2BiU6DYMVkAkqgAhg@mail.gmail.com> In-Reply-To: <CAAz%2B7vqLgd5GSBfFMdD-xsAsEoujgPh8ZdKY4xZ1LO0h30OmSQ@mail.gmail.com> References: <CAAz%2B7vqLgd5GSBfFMdD-xsAsEoujgPh8ZdKY4xZ1LO0h30OmSQ@mail.gmail.com>
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On Fri, Jul 29, 2016 at 6:11 AM, Adam Starak <starak.adam@gmail.com> wrote: > Hello! > > My name is Adam. I participate in Google Summer of Code this year. I came > up with a big problem, which doesn't allow me to go further in my project= . > > I made a new syscall, which is going to retrieve sysctl data and put it > inside the nvlist. And here my problem is. I need to move somehow this da= ta > (packed nvlist) into the user space. Is there any chance to pass data fro= m > kernel to user space without knowing the size of it? > > Right now, the implementation of __sysctl() function requests void pointe= r > and size in order to get data. If allocated memory is too low, it returns > ENOMEM and you need to realloc the data. I wanted to avoid this situation= . Hey Adam, That is the usual way to do it. Just curious =E2=80=94 why do you want to avoid that situation? Your other option might be to put an upper limit on the size of the result, and pass a buffer of that size in from userspace. But then you are artificially limited to some arbitrary size and must preallocate a large buffer even in the case that the output is small. Best, Conrad
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