Re: [PATCH v2] riscv: Map the kernel with correct permissions the first time

[Alex Ghiti]

Le 27/05/2021 à 08:35, Christoph Hellwig a écrit :
> On Wed, May 26, 2021 at 03:41:10PM +0200, Alexandre Ghiti wrote:
> >   #ifdef CONFIG_64BIT
> > +#define is_kernel_mapping(x)	((x) >= kernel_virt_addr && (x) < (kernel_virt_addr + load_sz))
> > +#define is_linear_mapping(x)	((x) >= PAGE_OFFSET && (x) < kernel_virt_addr)
> > +
>
> Overly long lines.  Independ of that complex macros are generally much
> more readable if they are written more function-like, that is the name
> and paramtes are kept on a line of their own:
>
> #define is_kernel_mapping(x) \
> 	((x) >= kernel_virt_addr && (x) < (kernel_virt_addr + load_sz))
>
> But what is the reason to not make them type-safe inline functions
> anyway?
>
> >   #define __va_to_pa_nodebug(x)	({						\
> >   	unsigned long _x = x;							\
> > -	(_x < kernel_virt_addr) ?						\
> > +	is_linear_mapping(_x) ?							\
> >   		linear_mapping_va_to_pa(_x) : kernel_mapping_va_to_pa(_x);	\
> >   	})
>
> ... especially for something complex like this.

Turning those macros into inline functions gave me a hard time because 
of the XIP fixups that use macros to redefine symbols that should be 
accessed in RAM instead of flash before the MMU is enabled, I couldn't 
manage to get rid of header circular dependencies.

But, I think I finally found a solution to eliminate the need for those 
fixups. So for the moment, I'll send a v3 that fixes all your comments 
but this one and then I will work on this solution.

Thanks again for your comments,

Alex

> > +static inline bool is_va_kernel_lm_alias_text(uintptr_t va)
> > +{
> > +	return (va >= (uintptr_t)lm_alias(_start) && va < (uintptr_t)lm_alias(__init_text_begin));
>
> Overly long line as well.  And useless braces.
>
> > +static inline bool is_va_kernel_init_text(uintptr_t va)
> > +{
> > +	return (va >= (uintptr_t)__init_text_begin && va < (uintptr_t)__init_data_begin);
> > +}
>
> Same here.
>
> > +#ifdef CONFIG_STRICT_KERNEL_RWX
> > +static __init pgprot_t pgprot_from_va(uintptr_t va)
> > +{
> > +#ifdef CONFIG_64BIT
> > +	if (is_va_kernel_text(va) || is_va_kernel_init_text(va))
> > +		return PAGE_KERNEL_READ_EXEC;
> > +
> > +	/*
> > +	 * We must mark only text as read-only as init text will get freed later
> > +	 * and rodata section is marked readonly in mark_rodata_ro.
> > +	 */
> > +	if (is_va_kernel_lm_alias_text(va))
> > +		return PAGE_KERNEL_READ;
> > +
> > +	return PAGE_KERNEL;
> > +#else
> > +	if (is_va_kernel_text(va))
> > +		return PAGE_KERNEL_READ_EXEC;
> > +
> > +	if (is_va_kernel_init_text(va))
> > +		return PAGE_KERNEL_EXEC;
> > +
> > +	return PAGE_KERNEL;
> > +#endif /* CONFIG_64BIT */
> > +}
>
> If the entire function is different for config symbols please just
> split it into two separate functions.  But to make the difference more
> clear IS_ENABLED might fit better here:
>
> static __init pgprot_t pgprot_from_va(uintptr_t va)
> {
> 	if (is_va_kernel_text(va))
> 		return PAGE_KERNEL_READ_EXEC;
> 	if (is_va_kernel_init_text(va))
> 		return IS_ENABLED(CONFIG_64BIT) ?
> 			PAGE_KERNEL_READ_EXEC : PAGE_KERNEL_EXEC;
> 	if (IS_ENABLED(CONFIG_64BIT) && is_va_kernel_lm_alias_text(va))
> 		return PAGE_KERNEL_READ;
> 	return PAGE_KERNEL;
> }
>
> Preferable with comments explaining the 32-bit vs 64-bit difference.
>
> > +void mark_rodata_ro(void)
> > +{
> > +	unsigned long rodata_start = (unsigned long)__start_rodata;
> > +	unsigned long data_start = (unsigned long)_data;
> > +	unsigned long __maybe_unused lm_rodata_start = (unsigned long)lm_alias(__start_rodata);
> > +	unsigned long __maybe_unused lm_data_start = (unsigned long)lm_alias(_data);
> > +
> > +	set_memory_ro(rodata_start, (data_start - rodata_start) >> PAGE_SHIFT);
> > +#ifdef CONFIG_64BIT
> > +	set_memory_ro(lm_rodata_start, (lm_data_start - lm_rodata_start) >> PAGE_SHIFT);
> > +#endif
>
> Lots of unreadable overly lone lines.  Why not add a helper and do
> something like:
>
> static void set_kernel_memory_ro(char *startp, char *endp)
> {
>          unsigned long start = (unsigned long)startp;
> 	unsigned long end = (unsigned long)endp;
>
> 	set_memory_ro(start, (start - end) >> PAGE_SHIFT);
> }
>
>          set_kernel_memory_ro(_start_rodata, _data);
> 	if (IS_ENABLED(CONFIG_64BIT))
> 		set_kernel_memory_ro(lm_alias(__start_rodata), lm_alias(_data));
>
>
> > +static __init pgprot_t pgprot_from_va(uintptr_t va)
> > +{
> > +#ifdef CONFIG_64BIT
> > +	if (is_kernel_mapping(va))
> > +		return PAGE_KERNEL_EXEC;
> > +
> > +	if (is_linear_mapping(va))
> > +		return PAGE_KERNEL;
> > +
> > +	return PAGE_KERNEL;
> > +#else
> > +	return PAGE_KERNEL_EXEC;
> > +#endif /* CONFIG_64BIT */
> > +}
> > +#endif /* CONFIG_STRICT_KERNEL_RWX */
> > +
>
> Same comment as for the other version.  This could become:
>
> static __init pgprot_t pgprot_from_va(uintptr_t va)
> {
> 	if (IS_ENABLED(CONFIG_64BIT) && !is_kernel_mapping(va))
> 		return PAGE_KERNEL;
> 	return PAGE_KERNEL_EXEC;
> }
>
> > -static void __init create_kernel_page_table(pgd_t *pgdir, uintptr_t map_size)
> > +static void __init create_kernel_page_table(pgd_t *pgdir, uintptr_t map_size, bool early)
>
> Overly long line.
>
> >   	for (va = kernel_virt_addr; va < end_va; va += map_size)
> >   		create_pgd_mapping(pgdir, va,
> >   				   load_pa + (va - kernel_virt_addr),
> > -				   map_size, PAGE_KERNEL_EXEC);
> > +				   map_size, early ? PAGE_KERNEL_EXEC : pgprot_from_va(va));
>
> Same here.  But why not pass in a "pgprot_t ram_pgprot" instead of the
> bool, which would be self-documenting.
>
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