elf.5: Document PN_XNUM extension

In linux-2.6.34-rc1, an ELF core extension was added; user-land tools
manipulating ELF core dump such as gdb and binutils has already been
modified before; so elf.5 needs to be modified accordingly.

You can follow information on the ELF extension via the LKML post:
http://lkml.org/lkml/2010/1/3/103
Date	Mon, 04 Jan 2010 10:06:07 +0900 (JST)
Subject	[RESEND][mmotm][PATCH v2, 0/5] elf coredump: Add extended numbering support

This Linux-specific extension was added in kernel 2.6.34.

Reviewed-by: Petr Baudis <pasky@suse.cz>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man5/elf.5

@@ -546,6 +546,30 @@ in bytes.
 If a file has no program header,
 .IR e_phnum
 holds the value zero.
+.IP
+If the number of entries in the program header table is larger than or equal to
+.\" This is a Linux extension, added in Linux 2.6.34.
+.BR PN_XNUM
+(0xffff), this member holds
+.BR PN_XNUM
+(0xffff) and the real number of entries in the program header table is held
+in the
+.IR sh_info
+member of the initial entry in section header table.
+Otherwise, the
+.IR sh_info
+member of the initial entry contains the value zero.
+.\" .Bl -tag -width "PN_XNUM"
+.RS 12
+.TP 9
+.BR PN_XNUM
+This is defined as 0xffff, the largest number
+.IR e_phnum
+can have, specifying where the actual number of program headers is assigned.
+.PD
+.RE
+.\" .El
+.IP
 .TP
 .IR e_shentsize
 This member holds a sections header's size in bytes.
@@ -564,6 +588,18 @@ If a file has no section
 header table,
 .IR e_shnum
 holds the value of zero.
+.IP
+If the number of entries in the section header table is larger than or equal to
+.BR SHN_LORESERVE
+(0xff00),
+.IR e_shnum
+holds the value zero and the real number of entries in the section header
+table is held in the
+.IR sh_size
+member of the initial entry in section header table.
+Otherwise, the
+.IR sh_size
+member of the initial entry in the section header table holds the value zero.
 .TP
 .IR e_shstrndx
 This member holds the section header table index of the entry associated
@@ -571,6 +607,17 @@ with the section name string table.
 If the file has no section name string
 table, this member holds the value
 .BR SHN_UNDEF .
+.IP
+If the index of section name string table section is larger than or equal to
+.BR SHN_LORESERVE
+(0xff00), this member holds
+.BR SHN_XINDEX
+(0xffff) and the real index of the section name string table section
+is held in the
+.IR sh_link
+member of the initial entry in section header table. Otherwise, the
+.IR sh_link
+member of the initial entry in section header table contains the value zero.
 .RS 12
 .\" .Bl -tag -width "SHN_LORESERVE"
 .TP 14
@@ -844,7 +891,17 @@ holds the size in bytes of each entry.
 .PP
 A section header table index is a subscript into this array.
 Some section
-header table indices are reserved.
+header table indices are reserved:
+the initial entry and the indices between
+.B SHN_LORESERVE
+and
+.BR SHN_HIRESERVE .
+The initial entry is used in ELF extensions for
+.IR e_phnum ,
+.IR e_shnum
+and
+.IR e_strndx ;
+in other cases, each field in the initial entry is set to zero.
 An object file does not have sections for
 these special indices:
 .\" .Bl -tag -width "SHN_LORESERVE"
@@ -1856,6 +1913,16 @@ This is automatically populated by the linker.
 ELF first appeared in
 System V.
 The ELF format is an adopted standard.
+.PP
+The extensions for
+.IR e_phnum ,
+.IR e_shnum
+and
+.IR e_strndx
+respectively are
+Linux extensions.
+Sun, BSD and AMD64 also support them; for further information,
+look under SEE ALSO.
 .\" .SH AUTHORS
 .\" The original version of this manual page was written by
 .\" .An Jeroen Ruigrok van der Werven
@@ -1881,3 +1948,10 @@ Santa Cruz Operation,
 Unix System Laboratories,
 "Object Files",
 .IR "Executable and Linking Format (ELF)" .
+.PP
+Sun Microsystems,
+.IR "Linker and Libraries Guide" .
+.PP
+AMD64 ABI Draft,
+.IR "System V Application Binary Interface AMD64 Architecture Processor Supplement" .
+.PP