Re: [PATCH] kprobes: Enable tracing for mololithic kernel images

[Song Liu <song@kernel.org>]

con <will@kernel.org>, Masahiro Yamada <masahiroy@kernel.org>, Jarkko Sakkinen <jarkko@profian.com>, Sami Tolvanen <samitolvanen@google.com>, "Naveen N. Rao" <naveen.n.rao@linux.ibm.com>, Marco Elver <elver@google.com>, Kees Cook <keescook@chromium.org>, Steven Rostedt <rostedt@goodmis.org>, Nathan Chancellor <nathan@kernel.org>, "Russell King \(Oracle\)" <rmk+kernel@armlinux.org.uk>, Mark Brown <broonie@kernel.org>, Borislav Petkov <bp@alien8.de>, Alexander Egorenkov <egorenar@linux.ibm.com>, Thomas Bogendoerfer <tsbogend@alpha.franken.de>, Parisc List <linux-parisc@vger.kernel.org>, Nathaniel McCallum <nathaniel@profian.com>, Dmitry Torokhov <dmitry.torokhov@gmail.com>, "David S. Miller" <davem@davemloft.net>, "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>, Tobias Huschle <huschle@linux.ibm.com>, "Peter Zijlstra \(Intel\)" <peterz@infradead.org>, "H. Peter Anvin" <hpa@zytor.com>, sparclinux <sparclinux@vger.kernel.org>, Tiezhu Yang <yangtiezhu@loongson.cn>, Miroslav Benes <
 mbenes@suse.cz>, Chen Zhongjin <chenzhongjin@huawei.com>, Ard Biesheuvel <ardb@kernel.org>, the arch/x86 maintainers <x86@kernel.org>, Russell King <linux@armlinux.org.uk>, linux-riscv <linux-riscv@lists.infradead.org>, Ingo Molnar <mingo@redhat.com>, Aaron Tomlin <atomlin@redhat.com>, Albert Ou <aou@eecs.berkeley.edu>, Heiko Carstens <hca@linux.ibm.com>, Liao Chang <liaochang1@huawei.com>, Paul Walmsley <paul.walmsley@sifive.com>, Josh Poimboeuf <jpoimboe@kernel.org>, Thomas Richter <tmricht@linux.ibm.com>, "open list:BROADCOM NVRAM DRIVER" <linux-mips@vger.kernel.org>, Changbin Du <changbin.du@intel.com>, Palmer Dabbelt <palmer@dabbelt.com>, linuxppc-dev <linuxppc-dev@lists.ozlabs.org>, "linux-modules@vger.kernel.org" <linux-modules@vger.kernel.org>
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On Thu, Jun 9, 2022 at 1:34 AM Christophe Leroy
<christophe.leroy@csgroup.eu> wrote:
>
>
>
> Le 08/06/2022 à 18:12, Song Liu a écrit :
> > On Wed, Jun 8, 2022 at 7:21 AM Masami Hiramatsu <mhiramat@kernel.org> wrote:
> >>
> >> Hi Jarkko,
> >>
> >> On Wed, 8 Jun 2022 08:25:38 +0300
> >> Jarkko Sakkinen <jarkko@kernel.org> wrote:
> >>
> >>> On Wed, Jun 08, 2022 at 10:35:42AM +0800, Guo Ren wrote:
> >>>> .
> >>>>
> >>>> On Wed, Jun 8, 2022 at 8:02 AM Jarkko Sakkinen <jarkko@profian.com> wrote:
> >>>>>
> >>>>> Tracing with kprobes while running a monolithic kernel is currently
> >>>>> impossible because CONFIG_KPROBES is dependent of CONFIG_MODULES.  This
> >>>>> dependency is a result of kprobes code using the module allocator for the
> >>>>> trampoline code.
> >>>>>
> >>>>> Detaching kprobes from modules helps to squeeze down the user space,
> >>>>> e.g. when developing new core kernel features, while still having all
> >>>>> the nice tracing capabilities.
> >>>>>
> >>>>> For kernel/ and arch/*, move module_alloc() and module_memfree() to
> >>>>> module_alloc.c, and compile as part of vmlinux when either CONFIG_MODULES
> >>>>> or CONFIG_KPROBES is enabled.  In addition, flag kernel module specific
> >>>>> code with CONFIG_MODULES.
> >>>>>
> >>>>> As the result, kprobes can be used with a monolithic kernel.
> >>>> It's strange when MODULES is n, but vmlinux still obtains module_alloc.
> >>>>
> >>>> Maybe we need a kprobe_alloc, right?
> >>>
> >>> Perhaps not the best name but at least it documents the fact that
> >>> they use the same allocator.
> >>>
> >>> Few years ago I carved up something "half-way there" for kprobes,
> >>> and I used the name text_alloc() [*].
> >>>
> >>> [*] https://lore.kernel.org/all/20200724050553.1724168-1-jarkko.sakkinen@linux.intel.com/
> >>
> >> Yeah, I remember that. Thank you for updating your patch!
> >> I think the idea (split module_alloc() from CONFIG_MODULE) is good to me.
> >> If module support maintainers think this name is not good, you may be
> >> able to rename it as text_alloc() and make the module_alloc() as a
> >> wrapper of it.
> >
> > IIUC, most users of module_alloc() use it to allocate memory for text, except
> > that module code uses it for both text and data. Therefore, I guess calling it
> > text_alloc() is not 100% accurate until we change the module code (to use
> > a different API to allocate memory for data).
>
> When CONFIG_ARCH_WANTS_MODULES_DATA_IN_VMALLOC, module code uses
> module_alloc() for text and vmalloc() for data, see function
> move_module() in kernel/module/main.c

Thanks for the pointer! I will play with it.

Song