linux-kernel - Re: [PATCH] powerpc64/ftrace: fix module loading without patchable function entries

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Message-ID: <wlscshbqan2svtqkz5xc6v47tzndfnsvge7h4lbfn67zoplekl@elt2oxwhrp6f>
Date: Thu, 13 Feb 2025 22:29:45 +0530
From: Naveen N Rao <naveen@...nel.org>
To: Anthony Iliopoulos <ailiop@...e.com>
Cc: linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] powerpc64/ftrace: fix module loading without patchable
 function entries

On Wed, Feb 05, 2025 at 12:18:21AM +0100, Anthony Iliopoulos wrote:
> get_stubs_size assumes that there must always be at least one patchable
> function entry, which is not always the case (modules that export data
> but no code), otherwise it returns -ENOEXEC and thus the section header
> sh_size is set to that value. During module_memory_alloc() the size is
> passed to execmem_alloc() after being page-aligned and thus set to zero
> which will cause it to fail the allocation (and thus module loading) as
> __vmalloc_node_range() checks for zero-sized allocs and returns null:
> 
> [  115.466896] module_64: cast_common: doesn't contain __patchable_function_entries.
> [  115.469189] ------------[ cut here ]------------
> [  115.469496] WARNING: CPU: 0 PID: 274 at mm/vmalloc.c:3778 __vmalloc_node_range_noprof+0x8b4/0x8f0
> ...
> [  115.478574] ---[ end trace 0000000000000000 ]---
> [  115.479545] execmem: unable to allocate memory

Ugh, that's nasty.

> 
> Fix this by removing the check completely, since it is anyway not
> helpful to propagate this as an error upwards.
> 
> Fixes: eec37961a56a ("powerpc64/ftrace: Move ftrace sequence out of line")
> Signed-off-by: Anthony Iliopoulos <ailiop@...e.com>
> ---
>  arch/powerpc/kernel/module_64.c | 4 ----
>  1 file changed, 4 deletions(-)
> 

Thanks for fixing this. It might also be good to add a check in 
setup_ftrace_ool_stubs(). Something like this:

diff --git a/arch/powerpc/kernel/module_64.c b/arch/powerpc/kernel/module_64.c
index 34a5aec4908f..c10a9c66cfe3 100644
--- a/arch/powerpc/kernel/module_64.c
+++ b/arch/powerpc/kernel/module_64.c
@@ -1125,6 +1125,10 @@ static int setup_ftrace_ool_stubs(const Elf64_Shdr *sechdrs, unsigned long addr,
        unsigned int i, total_stubs, num_stubs;
        struct ppc64_stub_entry *stub;
 
+       /* Bail out early if no traceable functions */
+       if (!me->arch.ool_stub_count)
+               return 0;
+
        total_stubs = sechdrs[me->arch.stubs_section].sh_size / sizeof(*stub);
        num_stubs = roundup(me->arch.ool_stub_count * sizeof(struct ftrace_ool_stub),
                            sizeof(struct ppc64_stub_entry)) / sizeof(struct ppc64_stub_entry);


Regardless of that, for this patch:
Acked-by: Naveen N Rao (AMD) <naveen@...nel.org>


- Naveen