Re: [PATCH v5 2/2] arch/riscv: Enable kprobes when CONFIG_MODULES=n

["Jarkko Sakkinen" <jarkko@kernel.org>]

On Tue Mar 26, 2024 at 6:49 PM EET, Jarkko Sakkinen wrote:
> On Tue Mar 26, 2024 at 3:57 PM EET, Alexandre Ghiti wrote:
> > Hi Jarkko,
> >
> > On 25/03/2024 22:55, Jarkko Sakkinen wrote:
> > > Tacing with kprobes while running a monolithic kernel is currently
> > > impossible due the kernel module allocator dependency.
> > >
> > > Address the issue by implementing textmem API for RISC-V.
> > >
> > > Link: https://www.sochub.fi # for power on testing new SoC's with a minimal stack
> > > Link: https://lore.kernel.org/all/20220608000014.3054333-1-jarkko@profian.com/ # continuation
> > > Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
> > > ---
> > > v5:
> > > - No changes, expect removing alloc_execmem() call which should have
> > >    been part of the previous patch.
> > > v4:
> > > - Include linux/execmem.h.
> > > v3:
> > > - Architecture independent parts have been split to separate patches.
> > > - Do not change arch/riscv/kernel/module.c as it is out of scope for
> > >    this patch set now.
> > > v2:
> > > - Better late than never right? :-)
> > > - Focus only to RISC-V for now to make the patch more digestable. This
> > >    is the arch where I use the patch on a daily basis to help with QA.
> > > - Introduce HAVE_KPROBES_ALLOC flag to help with more gradual migration.
> > > ---
> > >   arch/riscv/Kconfig          |  1 +
> > >   arch/riscv/kernel/Makefile  |  3 +++
> > >   arch/riscv/kernel/execmem.c | 22 ++++++++++++++++++++++
> > >   3 files changed, 26 insertions(+)
> > >   create mode 100644 arch/riscv/kernel/execmem.c
> > >
> > > diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> > > index e3142ce531a0..499512fb17ff 100644
> > > --- a/arch/riscv/Kconfig
> > > +++ b/arch/riscv/Kconfig
> > > @@ -132,6 +132,7 @@ config RISCV
> > >   	select HAVE_KPROBES if !XIP_KERNEL
> > >   	select HAVE_KPROBES_ON_FTRACE if !XIP_KERNEL
> > >   	select HAVE_KRETPROBES if !XIP_KERNEL
> > > +	select HAVE_ALLOC_EXECMEM if !XIP_KERNEL
> > >   	# https://github.com/ClangBuiltLinux/linux/issues/1881
> > >   	select HAVE_LD_DEAD_CODE_DATA_ELIMINATION if !LD_IS_LLD
> > >   	select HAVE_MOVE_PMD
> > > diff --git a/arch/riscv/kernel/Makefile b/arch/riscv/kernel/Makefile
> > > index 604d6bf7e476..337797f10d3e 100644
> > > --- a/arch/riscv/kernel/Makefile
> > > +++ b/arch/riscv/kernel/Makefile
> > > @@ -73,6 +73,9 @@ obj-$(CONFIG_SMP)		+= cpu_ops.o
> > >   
> > >   obj-$(CONFIG_RISCV_BOOT_SPINWAIT) += cpu_ops_spinwait.o
> > >   obj-$(CONFIG_MODULES)		+= module.o
> > > +ifeq ($(CONFIG_ALLOC_EXECMEM),y)
> > > +obj-y				+= execmem.o
> > > +endif
> > >   obj-$(CONFIG_MODULE_SECTIONS)	+= module-sections.o
> > >   
> > >   obj-$(CONFIG_CPU_PM)		+= suspend_entry.o suspend.o
> > > diff --git a/arch/riscv/kernel/execmem.c b/arch/riscv/kernel/execmem.c
> > > new file mode 100644
> > > index 000000000000..3e52522ead32
> > > --- /dev/null
> > > +++ b/arch/riscv/kernel/execmem.c
> > > @@ -0,0 +1,22 @@
> > > +// SPDX-License-Identifier: GPL-2.0-or-later
> > > +
> > > +#include <linux/mm.h>
> > > +#include <linux/execmem.h>
> > > +#include <linux/vmalloc.h>
> > > +#include <asm/sections.h>
> > > +
> > > +void *alloc_execmem(unsigned long size, gfp_t /* gfp */)
>
> Need to have the parameter name here. I guess this could just as well
> pass through gfp to vmalloc from the caller as kprobes does call
> module_alloc() with GFP_KERNEL set in RISC-V.
>
> > > +{
> > > +	return __vmalloc_node_range(size, 1, MODULES_VADDR,
> > > +				    MODULES_END, GFP_KERNEL,
> > > +				    PAGE_KERNEL, 0, NUMA_NO_NODE,
> > > +				    __builtin_return_address(0));
> > > +}
> >
> >
> > The __vmalloc_node_range() line ^^ must be from an old kernel since we 
> > added VM_FLUSH_RESET_PERMS in 6.8, see 749b94b08005 ("riscv: Fix 
> > module_alloc() that did not reset the linear mapping permissions").
> >
> > In addition, I guess module_alloc() should now use alloc_execmem() right?
>
> Ack for the first comment. For the 2nd it is up to arch/<arch> to choose
> whether to have shared or separate allocators.
>
> So if you want I can change it that way but did not want to make the
> call myself.
>
> >
> >
> > > +
> > > +void free_execmem(void *region)
> > > +{
> > > +	if (in_interrupt())
> > > +		pr_warn("In interrupt context: vmalloc may not work.\n");
> > > +
> > > +	vfree(region);
> > > +}
> >
> >
> > I remember Mike Rapoport sent a patchset to introduce an API for 
> > executable memory allocation 
> > (https://lore.kernel.org/linux-mm/20230918072955.2507221-1-rppt@kernel.org/), 
> > how does this intersect with your work? I don't know the status of his 
> > patchset though.
> >
> > Thanks,
> >
> > Alex
>
> I have also made a patch set for kprobes in the 2022:
>
> https://lore.kernel.org/all/20220608000014.3054333-1-jarkko@profian.com/
>
> I think this Calvin's, Mike's and my early patch set have the same
> problem: they try to choke all architectures at once. And further,
> Calvin's and Mike's work also try to cover also tracing subsystems
> at once.
>
> I feel that my relatively small patch set which deals only with
> trivial kprobe (which is more in the leaf than e.g. bpf which
> is more like orchestrator tool) and implements one arch of which
> dog food I actually eat is a better starting point.
>
> Arch code is always something where you need to have genuine
> understanding so full architecture coverage from day one is
> just too risky for stability. Linux is better off if people who
> work on a  specific arch proactively will "fill the holes".
>
> So the way I see my patch set is "lowest common denominator"
> in both architecture axis and tracing subsystem axist. It should
> not interfere that much with the other work (like bpf).

Here also there is a lot of kconfig flag logic changes. I've verified
them but still I think we are better off if this stuff is put in the
wild first in small scale rather than spraying kernel with code changes
in the first run.

BR, Jarkko

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