On Thu, 2022-09-01 at 07:01 +0000, Christophe Leroy wrote:
>
>
> Le 01/09/2022 à 07:58, Benjamin Gray a écrit :
> > From: Russell Currey <rus...@russell.cc>
> >
> > powerpc allocates a text poke area of one page that is used by
> > patch_instruction() to modify read-only text when STRICT_KERNEL_RWX
> > is enabled.
> >
> > patch_instruction() is only designed for instructions,
> > so writing data using the text poke area can only happen 4 bytes
> > at a time - each with a page map/unmap, pte flush and syncs.
> >
> > This patch introduces patch_memory(), implementing the same
> > interface as memcpy(), similar to x86's text_poke() and s390's
> > s390_kernel_write(). patch_memory() only needs to map the text
> > poke area once, unless the write would cross a page boundary.
> >
> > Signed-off-by: Russell Currey <rus...@russell.cc>
> > Signed-off-by: Benjamin Gray <bg...@linux.ibm.com>
> > ---
> > arch/powerpc/include/asm/code-patching.h | 1 +
> > arch/powerpc/lib/code-patching.c | 65
> > ++++++++++++++++++++++++
> > 2 files changed, 66 insertions(+)
> >
> > diff --git a/arch/powerpc/include/asm/code-patching.h
> > b/arch/powerpc/include/asm/code-patching.h
> > index 1c6316ec4b74..3de90748bce7 100644
> > --- a/arch/powerpc/include/asm/code-patching.h
> > +++ b/arch/powerpc/include/asm/code-patching.h
> > @@ -76,6 +76,7 @@ int create_cond_branch(ppc_inst_t *instr, const
> > u32 *addr,
> > int patch_branch(u32 *addr, unsigned long target, int flags);
> > int patch_instruction(u32 *addr, ppc_inst_t instr);
> > int raw_patch_instruction(u32 *addr, ppc_inst_t instr);
> > +void *patch_memory(void *dest, const void *src, size_t size);
> >
> > static inline unsigned long patch_site_addr(s32 *site)
> > {
> > diff --git a/arch/powerpc/lib/code-patching.c
> > b/arch/powerpc/lib/code-patching.c
> > index 6edf0697a526..0cca39af44cb 100644
> > --- a/arch/powerpc/lib/code-patching.c
> > +++ b/arch/powerpc/lib/code-patching.c
> > @@ -14,6 +14,7 @@
> > #include <asm/page.h>
> > #include <asm/code-patching.h>
> > #include <asm/inst.h>
> > +#include <asm/cacheflush.h>
> >
> > static int __patch_instruction(u32 *exec_addr, ppc_inst_t instr,
> > u32 *patch_addr)
> > {
> > @@ -183,6 +184,65 @@ static int do_patch_instruction(u32 *addr,
> > ppc_inst_t instr)
> >
> > return err;
> > }
> > +
> > +static int do_patch_memory(void *dest, const void *src, size_t
> > size)
> > +{
> > + int err;
> > + unsigned long text_poke_addr, patch_addr;
> > +
> > + text_poke_addr = (unsigned
> > long)__this_cpu_read(text_poke_area)->addr;
> > +
> > + err = map_patch_area(dest, text_poke_addr);
>
> This is not in line with the optimisation done by
> https://patchwork.ozlabs.org/project/linuxppc-dev/patch/20220815114840.1468656-1-...@ellerman.id.au/
This patch hasn't changed since last year, thanks for the pointer.
>
> > + if (err)
> > + return err;
> > +
> > + patch_addr = text_poke_addr + offset_in_page(dest);
> > + copy_to_kernel_nofault((u8 *)patch_addr, src, size);
>
> copy_to_kernel_nofault() has a performance cost.
>
> > +
> > + flush_icache_range(patch_addr, size);
>
> Is that needed ? We are patching data, not text.
It's necessary if it gets used to patch text, which it might. Maybe we
should add a variable and only flush if the caller thinks it's needed.
The comment below should be updated for that too.
> > + unmap_patch_area(text_poke_addr);
> > +
> > + return 0;
> > +}
> > +
> > +/**
> > + * patch_memory - write data using the text poke area
> > + *
> > + * @dest: destination address
> > + * @src: source address
> > + * @size: size in bytes
> > + *
> > + * like memcpy(), but using the text poke area. No atomicity
> > guarantees.
> > + * Do not use for instructions, use patch_instruction() instead.
> > + * Handles crossing page boundaries, though you shouldn't need to.
> > + *
> > + * Return value:
> > + * @dest
> > + **/
> > +void *patch_memory(void *dest, const void *src, size_t size)
> > +{
> > + int err;
> > + unsigned long flags;
> > + size_t written, write_size;
> > +
> > + // If the poke area isn't set up, it's early boot and we
> > can just memcpy.
> > + if (!this_cpu_read(text_poke_area))
> > + return memcpy(dest, src, size);
> > +
> > + for (written = 0; written < size; written += write_size) {
> > + // Write as much as possible without crossing a
> > page boundary.
> > + write_size = min_t(size_t, size - written,
> > + PAGE_SIZE - offset_in_page(dest
> > + written));
> > +
> > + local_irq_save(flags);
> > + err = do_patch_memory(dest + written, src +
> > written, write_size);
> > + local_irq_restore(flags);
> > + if (err)
> > + return ERR_PTR(err);
> > + }
> > +
> > + return dest;
> > +}
> > #else /* !CONFIG_STRICT_KERNEL_RWX */
> >
> > static int do_patch_instruction(u32 *addr, ppc_inst_t instr)
> > @@ -190,6 +250,11 @@ static int do_patch_instruction(u32 *addr,
> > ppc_inst_t instr)
> > return raw_patch_instruction(addr, instr);
> > }
> >
> > +void *patch_memory(void *dest, const void *src, size_t size)
> > +{
> > + return memcpy(dest, src, size);
>
> In do_patch_memory() you have flush_icache_range(patch_addr, size);
>
> If that's really needed there, why don't we need it here as well ?
Good point. I might make the arguments
(void *dest, const void *src, size_t size, bool text)
and only do the icache flush if text is true.
>
> > +}
> > +
> > #endif /* CONFIG_STRICT_KERNEL_RWX */
> >
> > __ro_after_init DEFINE_STATIC_KEY_FALSE(init_mem_is_free);
> > --
> > 2.37.2
