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lib/elf: Allow loading relocatable higher half kernels with low load addresses

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This commit is contained in:
mintsuki 2024-05-22 06:45:50 +02:00
parent 5b1cd9d01b
commit 860d1b1da2
3 changed files with 37 additions and 20 deletions
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3
PROTOCOL.md
View File

@ -149,7 +149,8 @@ inside said section.
## Entry memory layout
The protocol mandates kernels to load themselves at or above
`0xffffffff80000000`. Lower half kernels are *not supported*.
`0xffffffff80000000`. Lower half kernels are *not supported*. For relocatable kernels
asking to be loaded at address 0, a minimum slide of `0xffffffff80000000` is applied.
At handoff, the kernel will be properly loaded and mapped with appropriate
MMU permissions, as supervisor, at the requested virtual memory address (provided it is at

52
common/lib/elf.c
View File

@ -366,6 +366,8 @@ static void elf64_get_ranges(uint8_t *elf, uint64_t slide, struct elf_range **_r
panic(true, "elf: phdr_size < sizeof(struct elf64_phdr)");
}
bool is_reloc = elf64_is_relocatable(elf, hdr);
for (uint16_t i = 0; i < hdr->ph_num; i++) {
struct elf64_phdr *phdr = (void *)elf + (hdr->phoff + i * hdr->phdr_size);
@ -374,7 +376,9 @@ static void elf64_get_ranges(uint8_t *elf, uint64_t slide, struct elf_range **_r
}
if (phdr->p_vaddr < FIXED_HIGHER_HALF_OFFSET_64) {
continue;
if (!is_reloc || phdr->p_vaddr >= 0x80000000) {
continue;
}
}
ranges_count++;
@ -395,7 +399,9 @@ static void elf64_get_ranges(uint8_t *elf, uint64_t slide, struct elf_range **_r
}
if (phdr->p_vaddr < FIXED_HIGHER_HALF_OFFSET_64) {
continue;
if (!is_reloc || phdr->p_vaddr >= 0x80000000) {
continue;
}
}
uint64_t load_addr = phdr->p_vaddr + slide;
@ -443,6 +449,11 @@ bool elf64_load(uint8_t *elf, uint64_t *entry_point, uint64_t *_slide, uint32_t
if (is_reloc) {
*is_reloc = false;
}
if (elf64_is_relocatable(elf, hdr)) {
if (is_reloc) {
*is_reloc = true;
}
}
uint64_t slide = 0;
size_t try_count = 0;
@ -458,6 +469,8 @@ bool elf64_load(uint8_t *elf, uint64_t *entry_point, uint64_t *_slide, uint32_t
panic(true, "elf: phdr_size < sizeof(struct elf64_phdr)");
}
bool lower_to_higher = false;
uint64_t min_vaddr = (uint64_t)-1;
uint64_t max_vaddr = 0;
for (uint16_t i = 0; i < hdr->ph_num; i++) {
@ -467,9 +480,16 @@ bool elf64_load(uint8_t *elf, uint64_t *entry_point, uint64_t *_slide, uint32_t
continue;
}
// Drop entries not in the higher half
if (phdr->p_vaddr < FIXED_HIGHER_HALF_OFFSET_64) {
continue;
if (!*is_reloc || phdr->p_vaddr >= 0x80000000) {
continue;
}
lower_to_higher = true;
slide = FIXED_HIGHER_HALF_OFFSET_64;
} else {
if (lower_to_higher) {
panic(true, "elf: Mix of lower and higher half PHDRs");
}
}
// check for overlapping phdrs
@ -480,9 +500,10 @@ bool elf64_load(uint8_t *elf, uint64_t *entry_point, uint64_t *_slide, uint32_t
continue;
}
// Drop entries not in the higher half
if (phdr_in->p_vaddr < FIXED_HIGHER_HALF_OFFSET_64) {
continue;
if (!*is_reloc || phdr->p_vaddr >= 0x80000000) {
continue;
}
}
if (phdr_in == phdr) {
@ -524,8 +545,8 @@ bool elf64_load(uint8_t *elf, uint64_t *entry_point, uint64_t *_slide, uint32_t
}
}
if (max_vaddr == 0 || min_vaddr == (uint64_t)-1) {
panic(true, "elf: No higher half PHDRs exist");
if (min_vaddr == (uint64_t)-1) {
panic(true, "elf: No usable PHDRs exist");
}
image_size = max_vaddr - min_vaddr;
@ -537,17 +558,11 @@ bool elf64_load(uint8_t *elf, uint64_t *entry_point, uint64_t *_slide, uint32_t
*_image_size = image_size;
}
if (elf64_is_relocatable(elf, hdr)) {
if (is_reloc) {
*is_reloc = true;
}
}
again:
if (*is_reloc && kaslr) {
slide = rand32() & ~(max_align - 1);
slide = (rand32() & ~(max_align - 1)) + (lower_to_higher ? FIXED_HIGHER_HALF_OFFSET_64 : 0);
if ((*virtual_base - FIXED_HIGHER_HALF_OFFSET_64) + slide + image_size >= 0x80000000) {
if (*virtual_base + slide + image_size < 0xffffffff80000000 /* this comparison relies on overflow */) {
if (++try_count == max_simulated_tries) {
panic(true, "elf: Image wants to load too high");
}
@ -564,9 +579,10 @@ again:
continue;
}
// Drop entries not in the higher half
if (phdr->p_vaddr < FIXED_HIGHER_HALF_OFFSET_64) {
continue;
if (!*is_reloc || phdr->p_vaddr >= 0x80000000) {
continue;
}
}
// Sanity checks

2
test/linker.ld
View File

@ -8,7 +8,7 @@ PHDRS
SECTIONS
{
. = 0xffffffff80000000;
. = 0;
kernel_start = .;
.text : {