From 08b61c5d52077befb3471372f01b65bd40c4fd38 Mon Sep 17 00:00:00 2001
From: cem <cem@FreeBSD.org>
Date: Wed, 20 Jul 2016 16:59:36 +0000
Subject: [PATCH] Extend ELF coredump to support more than 65535 segments

The ELF e_phnum field is only 16 bits wide. To support more than 65535 segments
(program headers), Sun's "Linker and Libraries Guide" table 7-7 (or 12-7,
depending on document version) prescribes a special first section header where
sh_info represents the real number of program headers.

Test code to follow, when it is ready.

Reference:	http://docs.oracle.com/cd/E18752_01/pdf/817-1984.pdf

Reviewed by:	emaste, markj
Sponsored by:	EMC / Isilon Storage Division
Differential Revision:	https://reviews.freebsd.org/D7255
---
 sys/kern/imgact_elf.c | 37 ++++++++++++++++++++++++++++++++++---
 1 file changed, 34 insertions(+), 3 deletions(-)

diff --git a/sys/kern/imgact_elf.c b/sys/kern/imgact_elf.c
index a5934717d0d7..bce74753a808 100644
--- a/sys/kern/imgact_elf.c
+++ b/sys/kern/imgact_elf.c
@@ -1323,6 +1323,8 @@ __elfN(coredump)(struct thread *td, struct vnode *vp, off_t limit, int flags)
 	 * Collect info about the core file header area.
 	 */
 	hdrsize = sizeof(Elf_Ehdr) + sizeof(Elf_Phdr) * (1 + seginfo.count);
+	if (seginfo.count + 1 >= PN_XNUM)
+		hdrsize += sizeof(Elf_Shdr);
 	__elfN(prepare_notes)(td, &notelst, &notesz);
 	coresize = round_page(hdrsize + notesz) + seginfo.size;
 
@@ -1618,10 +1620,10 @@ __elfN(puthdr)(struct thread *td, void *hdr, size_t hdrsize, int numsegs,
 {
 	Elf_Ehdr *ehdr;
 	Elf_Phdr *phdr;
+	Elf_Shdr *shdr;
 	struct phdr_closure phc;
 
 	ehdr = (Elf_Ehdr *)hdr;
-	phdr = (Elf_Phdr *)((char *)hdr + sizeof(Elf_Ehdr));
 
 	ehdr->e_ident[EI_MAG0] = ELFMAG0;
 	ehdr->e_ident[EI_MAG1] = ELFMAG1;
@@ -1645,14 +1647,43 @@ __elfN(puthdr)(struct thread *td, void *hdr, size_t hdrsize, int numsegs,
 	ehdr->e_flags = 0;
 	ehdr->e_ehsize = sizeof(Elf_Ehdr);
 	ehdr->e_phentsize = sizeof(Elf_Phdr);
-	ehdr->e_phnum = numsegs + 1;
 	ehdr->e_shentsize = sizeof(Elf_Shdr);
-	ehdr->e_shnum = 0;
 	ehdr->e_shstrndx = SHN_UNDEF;
+	if (numsegs + 1 < PN_XNUM) {
+		ehdr->e_phnum = numsegs + 1;
+		ehdr->e_shnum = 0;
+	} else {
+		ehdr->e_phnum = PN_XNUM;
+		ehdr->e_shnum = 1;
+
+		ehdr->e_shoff = ehdr->e_phoff +
+		    (numsegs + 1) * ehdr->e_phentsize;
+		KASSERT(ehdr->e_shoff == hdrsize - sizeof(Elf_Shdr),
+		    ("e_shoff: %zu, hdrsize - shdr: %zu",
+		     ehdr->e_shoff, hdrsize - sizeof(Elf_Shdr)));
+
+		shdr = (Elf_Shdr *)((char *)hdr + ehdr->e_shoff);
+		memset(shdr, 0, sizeof(*shdr));
+		/*
+		 * A special first section is used to hold large segment and
+		 * section counts.  This was proposed by Sun Microsystems in
+		 * Solaris and has been adopted by Linux; the standard ELF
+		 * tools are already familiar with the technique.
+		 *
+		 * See table 7-7 of the Solaris "Linker and Libraries Guide"
+		 * (or 12-7 depending on the version of the document) for more
+		 * details.
+		 */
+		shdr->sh_type = SHT_NULL;
+		shdr->sh_size = ehdr->e_shnum;
+		shdr->sh_link = ehdr->e_shstrndx;
+		shdr->sh_info = numsegs + 1;
+	}
 
 	/*
 	 * Fill in the program header entries.
 	 */
+	phdr = (Elf_Phdr *)((char *)hdr + ehdr->e_phoff);
 
 	/* The note segement. */
 	phdr->p_type = PT_NOTE;