Re: [Question] About the elf program header size

[YinFengwei]

On Fri, Jun 27, 2025 at 09:35:45AM +0800, Kees Cook wrote:
> On Fri, Jun 27, 2025 at 09:04:11AM +0800, YinFengwei wrote:
> > We had a script generated assembly code. built it with gcc and the
> > output elf file had 78 program headers.
> 
> Why so many?
> 
> > On an arm64 platform, if we have 64KB base page size, the elf can
> > be started correctly. But if we have 4KB base page size, the elf
> > can NOT be started with:
> >     cannot execute binary file: Exec format error
> > 
> > Look at the function load_elf_phdrs():
> >         if (size == 0 || size > 65536 || size > ELF_MIN_ALIGN)
> > 	                goto out;
> > 
> > ELF_MIN_ALIGN is defined as PAGE_SIZE on arm64. Which can explain
> > above inconsistent behaviors (from user perspetive).
> > 
> > I didn't find the limitation definition in ELF spec(Maybe I missed
> > some obvious info there). If I remove "size > ELF_MIN_ALIGN", the
> > same elf can be started correctly even with 4KB page size.
> > 
> > So my question is why we limit the who program headers total size
> > to PAGE_SIZE? git history couldn't tell anything because the
> > limitation was introduced when whole linux kernel tree was migrated
> > to git. Is there a possible constrain on other architecture? Thanks.
> 
> Looking through
> https://git.kernel.org/pub/scm/linux/kernel/git/history/history.git
> (which doesn't have linked history, so you have to examine explicit "pre
> git" tags), I see:
> 
> 4779b38bcb96 ("[PATCH] Linux-0.99.13 (September 19, 1993)")
> Which says "ELF binary support it a notable change." Here, the PAGE_SIZE
> check does not exist. When ELF interp support was added in
> 9e11983a5a3e ("Import 0.99.15f"), we see the check appear, and I can
> find no rationale.
> 
> And with 6a8d38945cf4 ("binfmt_elf: Hoist ELF program header loading to
> a function"), the PAGE_SIZE check is _added_ for non-interp loads.
> 
> It seems the 64K count limit is sufficient? (If the goal was to avoid
> large memory allocations happening from userspace, we're way past
> PAGE_SIZE these days between IPC, BPF, etc.) Does this work for you?
> 
> 
> diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> index a43363d593e5..92de44b8765f 100644
> --- a/fs/binfmt_elf.c
> +++ b/fs/binfmt_elf.c
> @@ -519,7 +519,7 @@ static struct elf_phdr *load_elf_phdrs(const struct elfhdr *elf_ex,
>  	/* Sanity check the number of program headers... */
>  	/* ...and their total size. */
>  	size = sizeof(struct elf_phdr) * elf_ex->e_phnum;
> -	if (size == 0 || size > 65536 || size > ELF_MIN_ALIGN)
> +	if (size == 0 || size > 65536)
>  		goto out;
>  
>  	elf_phdata = kmalloc(size, GFP_KERNEL);
Just want to check: are you going to push this change to be merged?
Thanks.


Regards
Yin, Fengwei

> 
> 
> -- 
> Kees Cook