[Skiboot] [PATCH v2] Allow ELF executables with function descriptor entrypoint to be skiboot payloads

[Nathan Whitehorn]

Big-endian ELF64 ELF executables normally (the Linux kernel is an 
exception) have their entry point refer to a function descriptor instead 
of the first instruction. Distinguish between the Linux case and the 
function descriptor case, which is used for the FreeBSD kernel, by 
checking whether the entry point points into an executable section or 
not. This allows use of the FreeBSD kernel as a skiboot payload.

Signed-off-by: Nathan Whitehorn <nwhitehorn at freebsd.org>
---
  core/init.c   | 22 ++++++++++++++++++++++
  include/elf.h | 17 +++++++++++++++++
  2 files changed, 39 insertions(+)

diff --git a/core/init.c b/core/init.c
index 2c7e30c..e75b5a3 100644
--- a/core/init.c
+++ b/core/init.c
@@ -111,6 +111,7 @@ static bool try_load_elf64(struct elf_hdr *header)
  	struct elf64_hdr *kh = (struct elf64_hdr *)header;
  	uint64_t load_base = (uint64_t)kh;
  	struct elf64_phdr *ph;
+	struct elf64_shdr *sh;
  	unsigned int i;

  	/* Check it's a ppc64 LE ELF */
@@ -152,6 +153,27 @@ static bool try_load_elf64(struct elf_hdr *header)
  		prerror("INIT: Failed to find kernel entry !\n");
  		return false;
  	}
+
+	/* For the normal big-endian ELF ABI, the kernel entry points
+	 * to a function descriptor in the data section. Linux instead
+	 * has it point directly to code. Test whether it is pointing
+	 * into an executable section or not to figure this out. Default
+	 * to assuming it obeys the ABI.
+	 */
+	sh = (struct elf64_shdr *)(load_base + kh->e_shoff);
+	for (i = 0; i < kh->e_shnum; i++, sh++) {
+		if (sh->sh_addr > kh->e_entry ||
+		    (sh->sh_addr + sh->sh_size) <= kh->e_entry)
+			continue;
+
+		break;
+	}
+
+	if (i == kh->e_shnum || !(sh->sh_flags & ELF_SFLAGS_X)) {
+		kernel_entry = *(uint64_t *)(kernel_entry + load_base);
+		kernel_entry = kernel_entry - ph->p_vaddr + ph->p_offset;
+	}
+
  	kernel_entry += load_base;
  	kernel_32bit = false;

diff --git a/include/elf.h b/include/elf.h
index 0a52f3e..c600f7f 100644
--- a/include/elf.h
+++ b/include/elf.h
@@ -76,6 +76,23 @@ struct elf64_phdr {
  	uint64_t p_align;
  };

+/* 64-bit ELF section header */
+struct elf64_shdr {
+	uint32_t sh_name;
+	uint32_t sh_type;
+	uint64_t sh_flags;
+#define ELF_SFLAGS_X	0x4
+#define ELF_SFLAGS_A	0x2
+#define ELF_SFLAGS_W	0x1
+	uint64_t sh_addr;
+	uint64_t sh_offset;
+	uint64_t sh_size;
+	uint32_t sh_link;
+	uint32_t sh_info;
+	uint64_t sh_addralign;
+	uint64_t sh_entsize;
+};
+
  /* Some relocation related stuff used in relocate.c */
  struct elf64_dyn {
  	int64_t	 d_tag;