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

Christoph Hellwig
2021-05-27T06:36:37Z

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.

> +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.