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SubjectRe: [PATCH v2 4/6] kexec_file: Add mechanism to update kexec segments.
On Sat, 13 Aug 2016 00:18:23 -0300 Thiago Jung Bauermann <bauerman@linux.vnet.ibm.com> wrote:

> kexec_update_segment allows a given segment in kexec_image to have
> its contents updated. This is useful if the current kernel wants to
> send information to the next kernel that is up-to-date at the time of
> reboot.
>
> ...
>
> @@ -721,6 +721,105 @@ static struct page *kimage_alloc_page(struct kimage *image,
> return page;
> }
>
> +/**
> + * kexec_update_segment - update the contents of a kimage segment
> + * @buffer: New contents of the segment.
> + * @bufsz: @buffer size.
> + * @load_addr: Segment's physical address in the next kernel.
> + * @memsz: Segment size.
> + *
> + * This function assumes kexec_mutex is held.
> + *
> + * Return: 0 on success, negative errno on error.
> + */
> +int kexec_update_segment(const char *buffer, unsigned long bufsz,
> + unsigned long load_addr, unsigned long memsz)
> +{
> + int i;
> + unsigned long entry;
> + unsigned long *ptr = NULL;
> + void *dest = NULL;
> +
> + if (kexec_image == NULL) {
> + pr_err("Can't update segment: no kexec image loaded.\n");
> + return -EINVAL;
> + }
> +
> + /*
> + * kexec_add_buffer rounds up segment sizes to PAGE_SIZE, so
> + * we have to do it here as well.
> + */
> + memsz = ALIGN(memsz, PAGE_SIZE);
> +
> + for (i = 0; i < kexec_image->nr_segments; i++)
> + /* We only support updating whole segments. */
> + if (load_addr == kexec_image->segment[i].mem &&
> + memsz == kexec_image->segment[i].memsz) {
> + if (kexec_image->segment[i].do_checksum) {
> + pr_err("Trying to update non-modifiable segment.\n");
> + return -EINVAL;
> + }
> +
> + break;
> + }
> + if (i == kexec_image->nr_segments) {
> + pr_err("Couldn't find segment to update: 0x%lx, size 0x%lx\n",
> + load_addr, memsz);
> + return -EINVAL;
> + }
> +
> + for (entry = kexec_image->head; !(entry & IND_DONE) && memsz;
> + entry = *ptr++) {
> + void *addr = (void *) (entry & PAGE_MASK);
> +
> + switch (entry & IND_FLAGS) {
> + case IND_DESTINATION:
> + dest = addr;
> + break;
> + case IND_INDIRECTION:
> + ptr = __va(addr);
> + break;
> + case IND_SOURCE:
> + /* Shouldn't happen, but verify just to be safe. */
> + if (dest == NULL) {
> + pr_err("Invalid kexec entries list.");
> + return -EINVAL;
> + }
> +
> + if (dest == (void *) load_addr) {
> + struct page *page;
> + char *ptr;
> + size_t uchunk, mchunk;
> +
> + page = kmap_to_page(addr);
> +
> + ptr = kmap(page);

kmap_atomic() could be used here, and it is appreciably faster.


> + ptr += load_addr & ~PAGE_MASK;
> + mchunk = min_t(size_t, memsz,
> + PAGE_SIZE - (load_addr & ~PAGE_MASK));
> + uchunk = min(bufsz, mchunk);
> + memcpy(ptr, buffer, uchunk);
> +
> + kunmap(page);
> +
> + bufsz -= uchunk;
> + load_addr += mchunk;
> + buffer += mchunk;
> + memsz -= mchunk;
> + }
> + dest += PAGE_SIZE;
> + }
> +
> + /* Shouldn't happen, but verify just to be safe. */
> + if (ptr == NULL) {
> + pr_err("Invalid kexec entries list.");
> + return -EINVAL;
> + }
> + }
> +
> + return 0;
> +}
> +

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