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

Alex Ghiti
2021-05-28T08:24:56Z

Hi Christoph,

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?

No reason. I will then make those macros inline functions and send
another patchset to make the below macro an inline function too.

>
>> #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.
>
>> +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.

Ok.

>
>> +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.

checkpatch does not complain about those lines which are under 100
characters, what's the point in breaking them on multiple lines?

>
>> +#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.

Ok this is more compact, I'll do that with the comment.

>
>> +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));
>
>

Ok, that's better indeed. I will do something like that instead, to
avoid multiple versions of this helper:

int set_kernel_memory(char *startp, char *endp,

int (*set_memory)(unsigned long start, int
num_pages))

>> +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;
> }

Ok I'll do that.

>
>> -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.

This function is used to map the kernel mapping, the pgprot_t is then
different in create_kernel_page_table depending on the virtual address
so I can't pass a single pgprot_t for that or I would need a dummy
pgprot_t to test anyway.

Thank you for your review,

Alex