[Bug 295629] Linuxlator fails to run the linux x86_64 binary: Program headers not in the first page

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=3D295629

--- Comment #3 from Yuri Victorovich <[email protected]> ---
Here is the problem analysis:

 Root Cause

  The copilot binary has an extremely non-standard ELF layout where the pro=
gram
header table is placed at file offset 45,904,260 bytes (~44 MB into the fil=
e):

   Start of program headers:  45904260 (bytes into file)  # e_phoff =3D 0x2=
BC7184

  FreeBSD's ELF loader in sys/kern/imgact_elf.c enforces this check:

   static bool
   __elfN(phdr_in_zero_page)(const Elf_Ehdr *hdr)
   {
       return (hdr->e_phoff <=3D PAGE_SIZE &&
           (u_int)hdr->e_phentsize * hdr->e_phnum <=3D PAGE_SIZE - hdr->e_p=
hoff);
   }

  This requires:

   1. e_phoff =E2=89=A4 PAGE_SIZE (4096) =E2=80=94 FAILS: 45,904,260 >> 4,0=
96
   2. All program headers fit in the remainder of the first page

  The kernel only maps the first page of an executable into imgp->image_hea=
der
and immediately dereferences program headers from that buffer
(imgp->image_header + hdr->e_phoff). If e_phoff
  points beyond page 0, the data isn't there.

  Linux's kernel is different: load_elf_binary() uses kernel_read() to read
program headers at whatever offset e_phoff specifies, so it handles this la=
yout
fine.

  Why is the binary like this?

  The copilot binary (a Go binary) is built with debug info embedded, and h=
as
an unusual segment layout =E2=80=94 its first LOAD segment covers 0x3315b0 =
(~3.1 MB) of
read-only data placed before
  the executable code, with the program header table appended after most of=
 the
file content rather than near the ELF header. In a normal ELF binary, e_pho=
ff =3D
64 (immediately after the
  64-byte ELF header).

  Kernel fix: Extend FreeBSD's ELF loader to use vn_rdwr to read program
headers at arbitrary offsets (like Linux does), instead of requiring them in
the first page.

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