Re: [PATCH v3 1/8] scripts/sorttable: Handle RISC-V patchable ftrace entries

[Martin Kaiser <martin@kaiser.cx>]

Thus wrote Wang Han (wanghan@linux.alibaba.com):

> RISC-V uses -fpatchable-function-entry=8,4 when the compressed ISA is
> enabled and -fpatchable-function-entry=4,2 otherwise. In both cases, the
> patchable NOP area starts 8 bytes before the function symbol address.
> The __mcount_loc entries therefore point at the patchable NOP area
> associated with a function, while nm reports the function symbol at the
> entry address used for the function range check.

> After RISC-V selected HAVE_BUILDTIME_MCOUNT_SORT, sorttable started
> applying that range check at build time. Without allowing entries just
> before the reported function address, the mcount sorter treats valid
> RISC-V ftrace callsites as invalid weak-function entries and writes
> them back as zero. The resulting kernel boots with no ftrace entries,
> breaking dynamic ftrace and users such as livepatch.

> The failure is silent during the final link because zeroing weak-function
> entries is an expected sorttable operation. At boot, those zero entries
> are skipped by ftrace_process_locs(), so the only obvious symptom is that
> the vmlinux ftrace table has lost valid callsites and ftrace users cannot
> attach to them.

> CONFIG_FTRACE_SORT_STARTUP_TEST also reports the table as sorted in this
> state: it only checks that the __mcount_loc entries are in ascending
> order, which a fully zeroed table trivially satisfies. The original
> commit relied on this check and did not see the regression.

> On an affected RISC-V QEMU boot with both CONFIG_FTRACE_SORT_STARTUP_TEST
> and CONFIG_FTRACE_STARTUP_TEST enabled, the sort check still passes
> while ftrace reports zero usable entries and the early selftests fail:

>   [    0.000000] ftrace section at ffffffff8101da98 sorted properly
>   [    0.000000] ftrace: allocating 0 entries in 128 pages
>   [    0.054999] Testing tracer function: .. no entries found ..FAILED!
>   [    0.172407] tracer: function failed selftest, disabling
>   [    0.178186] Failed to init function_graph tracer, init returned -19

> Handle RISC-V like arm64 for the function-range check and allow
> patchable entries up to 8 bytes before the function address.

> With this fix, a RISC-V QEMU smoke boot with ftrace startup tests shows
> the vmlinux ftrace table is populated and dynamic ftrace still works:

>   [    0.000000] ftrace: allocating 46749 entries in 184 pages
>   [    0.051115] Testing tracer function: PASSED
>   [    1.283782] Testing dynamic ftrace: PASSED
>   [    6.275456] Testing tracer function_graph: PASSED

> Fixes: 0ca1724b56af ("riscv: ftrace: select HAVE_BUILDTIME_MCOUNT_SORT")
> Suggested-by: Steven Rostedt (Google) <rostedt@goodmis.org>
> Reviewed-by: Steven Rostedt <rostedt@goodmis.org>
> Reviewed-by: Shuai Xue <xueshuai@linux.alibaba.com>
> Reviewed-by: Chen Pei <cp0613@linux.alibaba.com>
> Link: https://lore.kernel.org/all/20260527113028.4b21a5de@fedora/
> Signed-off-by: Wang Han <wanghan@linux.alibaba.com>
> ---
>  scripts/sorttable.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)

> diff --git a/scripts/sorttable.c b/scripts/sorttable.c
> index e8ed11c680c6..d8dc2a1b7c31 100644
> --- a/scripts/sorttable.c
> +++ b/scripts/sorttable.c
> @@ -891,17 +891,22 @@ static int do_file(char const *const fname, void *addr)
>  	table_sort_t custom_sort = NULL;

>  	switch (elf_map_machine(ehdr)) {
> -	case EM_AARCH64:
>  #ifdef MCOUNT_SORT_ENABLED
> +	case EM_AARCH64:
> +		/* arm64 also needs RELA-based weak-function fixups. */
>  		sort_reloc = true;
>  		rela_type = 0x403;
> -		/* arm64 uses patchable function entry placing before function */
> +		/* fallthrough */
> +	case EM_RISCV:
> +		/* arm64 and RISC-V place patchable entries before the function. */
>  		before_func = 8;
> +#else
> +	case EM_AARCH64:
> +	case EM_RISCV:
>  #endif
>  		/* fallthrough */
>  	case EM_386:
>  	case EM_LOONGARCH:
> -	case EM_RISCV:
>  	case EM_S390:
>  	case EM_X86_64:
>  		custom_sort = sort_relative_table_with_data;
> -- 
> 2.43.0

I ran into this problem and came up with pretty much the same fix.

Reviewed-by: Martin Kaiser <martin@kaiser.cx>