Re: [PATCH] MIPS: ftrace: Fix memory corruption when kernel is located beyond 32 bits

[Thomas Bogendoerfer <tsbogend@alpha.franken.de>]

On Fri, Nov 28, 2025 at 09:30:06AM +0100, Gregory CLEMENT wrote:
> Since commit e424054000878 ("MIPS: Tracing: Reduce the overhead of
> dynamic Function Tracer"), the macro UASM_i_LA_mostly has been used,
> and this macro can generate more than 2 instructions. At the same
> time, the code in ftrace assumes that no more than 2 instructions can
> be generated, which is why it stores them in an int[2] array. However,
> as previously noted, the macro UASM_i_LA_mostly (and now UASM_i_LA)
> causes a buffer overflow when _mcount is beyond 32 bits. This leads to
> corruption of the variables located in the __read_mostly section.
> 
> This corruption was observed because the variable
> __cpu_primary_thread_mask was corrupted, causing a hang very early
> during boot.
> 
> This fix prevents the corruption by avoiding the generation of
> instructions if they could exceed 2 instructions in
> length. Fortunately, insn_la_mcount is only used if the instrumented
> code is located outside the kernel code section, so dynamic ftrace can
> still be used, albeit in a more limited scope. This is still
> preferable to corrupting memory and/or crashing the kernel.
> 
> Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>
> ---
> To go further and remove the limitations of dynamic trace support, the
> ftrace implementation for MIPS should be completely rewritten and
> inspired by what was done for arm64. This approach was chosen by
> Loongson: instead of trying to manage multiple instructions added on
> the fly, the support relies on a breakpoint, which is more robust.
> 
> However, this effort is significant, so I’ll leave it to those who are
> motivated to work on it. If needed, I can provide some guidance on the
> topic.
> ---
>  arch/mips/kernel/ftrace.c | 25 +++++++++++++++++++++----
>  1 file changed, 21 insertions(+), 4 deletions(-)

applied to mips-next.

Thomas.

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