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refactor linker to lay out ELF files and memory in the same way

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* module/system/vm/linker.scm (make-linker-object):
  (linker-object-section-symbol):
  (linker-object-symbols*): Create a symbol to the start of a linker
  object.  Hide it from the external linker-object-symbols* accessor.

  (segment-kind, count-segments): Sections without SHF_ALLOC don't get
  segments.
  (collate-objects-into-segments): Allow for #f segment types.  If two
  sections have the same type and flags, leave them in the same order.

  (align): Allow for 0 alignment.

  (add-elf-objects): New helper: puts the ELF data structures (header,
  segment table, and section table) in sections of their own.  This
  lends a nice clarity and conceptual unity to the linker.

  (relocate-section-header, allocate-segment): Lay out segments with
  congruent, contiguous addresses, so that we can just mmap the file and
  if debugging sections that are not in segments are present, they can
  be lazily paged in if needed by the kernel's VM system.

  (link-elf): Refactor to use the new interfaces.

* test-suite/tests/linker.test: Update to expect the additional sections
  for the header and section table.
This commit is contained in:
Andy Wingo 2013-04-28 09:31:28 +02:00
parent 51611a92f4
commit d4da9ba9c0
2 changed files with 357 additions and 169 deletions
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471
module/system/vm/linker.scm
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@ -82,7 +82,7 @@
linker-object-section
linker-object-bv
linker-object-relocs
linker-object-symbols
(linker-object-symbols* . linker-object-symbols)
make-string-table
string-table-intern
@ -90,6 +90,19 @@
link-elf))
(define-syntax fold-values
(lambda (x)
(syntax-case x ()
((_ proc list seed ...)
(with-syntax (((s ...) (generate-temporaries #'(seed ...))))
#'(let ((p proc))
(let lp ((l list) (s seed) ...)
(match l
(() (values s ...))
((elt . l)
(call-with-values (lambda () (p elt s ...))
(lambda (s ...) (lp l s ...))))))))))))
;; A relocation records a reference to a symbol. When the symbol is
;; resolved to an address, the reloc location will be updated to point
;; to the address.
@ -120,22 +133,43 @@
(address linker-symbol-address))
(define-record-type <linker-object>
(make-linker-object section bv relocs symbols)
(%make-linker-object section bv relocs symbols)
linker-object?
(section linker-object-section)
(bv linker-object-bv)
(relocs linker-object-relocs)
(symbols linker-object-symbols))
(define (make-linker-object section bv relocs symbols)
"Create a linker object with the @code{<elf-section>} header
@var{section}, bytevector contents @var{bv}, list of linker relocations
@var{relocs}, and list of linker symbols @var{symbols}."
(%make-linker-object section bv relocs
;; Hide a symbol to the beginning of the section
;; in the symbols.
(cons (make-linker-symbol (gensym "*section*") 0)
symbols)))
(define (linker-object-section-symbol object)
"Return the linker symbol corresponding to the start of this section."
(car (linker-object-symbols object)))
(define (linker-object-symbols* object)
"Return the linker symbols defined by the user for this this section."
(cdr (linker-object-symbols object)))
(define (make-string-table)
"Return a functional string table with one entry: the empty string."
'(("" 0 #vu8())))
(define (string-table-length table)
"Return the number of bytes needed for the string table @var{table}."
(let ((last (car table)))
;; The + 1 is for the trailing NUL byte.
(+ (cadr last) (bytevector-length (caddr last)) 1)))
(define (string-table-intern table str)
"Add @var{str} to the string table @var{table}. Yields two values: a
possibly newly allocated string table, and the byte index of the string
in that table."
(cond
((assoc str table)
=> (lambda (ent)
@ -147,6 +181,8 @@
next)))))
(define (link-string-table table)
"Link the functional string table @var{table} into a sequence of
bytes, suitable for use as the contents of an ELF string table section."
(let ((out (make-bytevector (string-table-length table) 0)))
(for-each
(lambda (ent)
@ -156,10 +192,15 @@
out))
(define (segment-kind section)
"Return the type of segment needed to store @var{section}, as a pair.
The car is the @code{PT_} segment type, or @code{#f} if the section
doesn't need to be present in a loadable segment. The cdr is a bitfield
of associated @code{PF_} permissions."
(let ((flags (elf-section-flags section)))
(cons (cond
((= (elf-section-type section) SHT_DYNAMIC) PT_DYNAMIC)
((zero? (logand SHF_ALLOC flags)) PT_NOTE)
;; Sections without SHF_ALLOC don't go in segments.
((zero? flags) #f)
(else PT_LOAD))
(logior (if (zero? (logand SHF_ALLOC flags))
0
@ -171,38 +212,65 @@
0
PF_W)))))
(define (count-segments objects)
"Return the total number of segments needed to represent the linker
objects in @var{objects}, including the segment needed for the ELF
header and segment table."
(length
(fold-values (lambda (object kinds)
(let ((kind (segment-kind (linker-object-section object))))
(if (and (car kind) (not (member kind kinds)))
(cons kind kinds)
kinds)))
objects
;; We know there will be at least one segment,
;; containing at least the header and segment table.
(list (cons PT_LOAD PF_R)))))
(define (group-by-cars ls)
(let lp ((in ls) (k #f) (group #f) (out '()))
(cond
((null? in)
(let lp ((ls ls) (k #f) (group #f) (out '()))
(match ls
(()
(reverse!
(if group
(cons (cons k (reverse! group)) out)
out)))
((and group (equal? k (caar in)))
(lp (cdr in) k (cons (cdar in) group) out))
(else
(lp (cdr in) (caar in) (list (cdar in))
(((k* . v) . ls)
(if (and group (equal? k k*))
(lp ls k (cons v group) out)
(lp ls k* (list v)
(if group
(cons (cons k (reverse! group)) out)
out))))))
out)))))))
(define (collate-objects-into-segments objects)
"Given the list of linker objects @var{objects}, group them into
contiguous ELF segments of the same type and flags. The result is an
alist that maps segment types to lists of linker objects. See
@code{segment-type} for a description of segment types. Within a
segment, the order of the linker objects is preserved."
(group-by-cars
(stable-sort!
(map (lambda (o)
(cons (segment-kind (linker-object-section o)) o))
objects)
(lambda (x y)
(let ((x-type (caar x)) (y-type (caar y))
(x-flags (cdar x)) (y-flags (cdar y))
(let* ((x-kind (car x)) (y-kind (car y))
(x-type (car x-kind)) (y-type (car y-kind))
(x-flags (cdr x-kind)) (y-flags (cdr y-kind))
(x-section (linker-object-section (cdr x)))
(y-section (linker-object-section (cdr y))))
(cond
((not (equal? x-kind y-kind))
(cond
((and x-type y-type)
(cond
((not (equal? x-flags y-flags))
(< x-flags y-flags))
((not (equal? x-type y-type))
(< x-type y-type))
(else
(< x-type y-type))))
(else
(not y-type))))
((not (equal? (elf-section-type x-section)
(elf-section-type y-section)))
(cond
@ -211,33 +279,25 @@
(else (< (elf-section-type x-section)
(elf-section-type y-section)))))
(else
(< (elf-section-size x-section)
(elf-section-size y-section)))))))))
;; Leave them in the initial order. This allows us to ensure
;; that the ELF header is written first.
#f)))))))
(define (align address alignment)
(if (zero? alignment)
address
(+ address
(modulo (- alignment (modulo address alignment)) alignment)))
(modulo (- alignment (modulo address alignment)) alignment))))
(define-syntax fold-values
(lambda (x)
(syntax-case x ()
((_ proc list seed ...)
(with-syntax (((s ...) (generate-temporaries #'(seed ...))))
#'(let ((p proc))
(let lp ((l list) (s seed) ...)
(match l
(() (values s ...))
((elt . l)
(call-with-values (lambda () (p elt s ...))
(lambda (s ...) (lp l s ...))))))))))))
(define (relocate-section-header sec fileaddr memaddr)
(define (relocate-section-header sec addr)
"Return a new section header, just like @var{sec} but with its
@code{addr} and @code{offset} set to @var{addr}."
(make-elf-section #:index (elf-section-index sec)
#:name (elf-section-name sec)
#:type (elf-section-type sec)
#:flags (elf-section-flags sec)
#:addr memaddr
#:offset fileaddr
#:addr addr
#:offset addr
#:size (elf-section-size sec)
#:link (elf-section-link sec)
#:info (elf-section-info sec)
@ -246,9 +306,10 @@
(define *page-size* 4096)
;; Adds object symbols to global table, relocating them from object
;; address space to memory address space.
(define (add-symbols symbols offset symtab)
"Add @var{symbols} to the symbol table @var{symtab}, relocating them
from object address space to memory address space. Returns a new symbol
table."
(fold-values
(lambda (symbol symtab)
(let ((name (linker-symbol-name symbol))
@ -259,90 +320,105 @@
symbols
symtab))
(define (alloc-segment phidx type flags objects
fileaddr memaddr symtab alignment)
(let* ((loadable? (not (zero? flags)))
(alignment (fold-values (lambda (o alignment)
(define (allocate-segment write-segment-header!
phidx type flags objects addr symtab alignment)
"Given a list of linker objects that should go in a segment, the type
and flags that the segment should have, and the address at which the
segment should start, compute the positions that each object should have
in the segment.
Returns three values: the address of the next byte after the segment, a
list of relocated objects, and the symbol table. The symbol table is
the same as @var{symtab}, augmented with the symbols defined in
@var{objects}, relocated to their positions in the image.
In what is something of a quirky interface, this routine also patches up
the segment table using @code{write-segment-header!}."
(let* ((alignment (fold-values (lambda (o alignment)
(lcm (elf-section-addralign
(linker-object-section o))
alignment))
objects
alignment))
(fileaddr (align fileaddr alignment))
(memaddr (align memaddr alignment)))
(receive (objects fileend memend symtab)
(addr (align addr alignment)))
(receive (objects endaddr symtab)
(fold-values
(lambda (o out fileaddr memaddr symtab)
(lambda (o out addr symtab)
(let* ((section (linker-object-section o))
(fileaddr
(if (= (elf-section-type section) SHT_NOBITS)
fileaddr
(align fileaddr (elf-section-addralign section))))
(memaddr
(align memaddr (elf-section-addralign section))))
(addr (align addr (elf-section-addralign section))))
(values
(cons (make-linker-object
(relocate-section-header section fileaddr
memaddr)
(relocate-section-header section addr)
(linker-object-bv o)
(linker-object-relocs o)
(linker-object-symbols o))
out)
(if (= (elf-section-type section) SHT_NOBITS)
fileaddr
(+ fileaddr (elf-section-size section)))
(+ memaddr (elf-section-size section))
(add-symbols (linker-object-symbols o) memaddr symtab))))
objects '() fileaddr memaddr symtab)
(values
(make-elf-segment #:index phidx
#:type type #:offset fileaddr
#:vaddr (if loadable? memaddr 0)
#:filesz (- fileend fileaddr)
#:memsz (if loadable? (- memend memaddr) 0)
#:flags flags #:align alignment)
(+ addr (elf-section-size section))
(add-symbols (linker-object-symbols o) addr symtab))))
objects
'() addr symtab)
(when type
(write-segment-header!
(make-elf-segment #:index phidx #:type type
#:offset addr #:vaddr addr
#:filesz (- endaddr addr) #:memsz (- endaddr addr)
#:flags flags #:align alignment)))
(values endaddr
(reverse objects)
symtab))))
(define (process-reloc reloc bv file-offset mem-offset symtab endianness)
(let ((ent (vhash-assq (linker-reloc-symbol reloc) symtab)))
(unless ent
(define (process-reloc reloc bv section-offset symtab endianness)
"Process a relocation. Given that a section containing @var{reloc}
was just written into the image @var{bv} at offset @var{section-offset},
fix it up so that its reference points to the correct position of its
symbol, as present in @var{symtab}."
(match (vhash-assq (linker-reloc-symbol reloc) symtab)
(#f
(error "Undefined symbol" (linker-reloc-symbol reloc)))
(let* ((file-loc (+ (linker-reloc-loc reloc) file-offset))
(mem-loc (+ (linker-reloc-loc reloc) mem-offset))
(addr (linker-symbol-address (cdr ent))))
((name . symbol)
;; The reloc was written at LOC bytes after SECTION-OFFSET.
(let* ((offset (+ (linker-reloc-loc reloc) section-offset))
(target (linker-symbol-address symbol)))
(case (linker-reloc-type reloc)
((rel32/4)
(let ((diff (- addr mem-loc)))
(let ((diff (- target offset)))
(unless (zero? (modulo diff 4))
(error "Bad offset" reloc symbol mem-offset))
(bytevector-s32-set! bv file-loc
(error "Bad offset" reloc symbol offset))
(bytevector-s32-set! bv offset
(+ (/ diff 4) (linker-reloc-addend reloc))
endianness)))
((abs32/1)
(bytevector-u32-set! bv file-loc addr endianness))
(bytevector-u32-set! bv offset target endianness))
((abs64/1)
(bytevector-u64-set! bv file-loc addr endianness))
(bytevector-u64-set! bv offset target endianness))
(else
(error "bad reloc type" reloc))))))
(error "bad reloc type" reloc)))))))
(define (write-linker-object bv o symtab endianness)
"Write the bytevector for the section wrapped by the linker object
@var{o} into the image @var{bv}. The section header in @var{o} should
already be relocated its final position in the image. Any relocations
in the section will be processed to point to the correct symbol
locations, as given in @var{symtab}."
(let* ((section (linker-object-section o))
(offset (elf-section-offset section))
(addr (elf-section-addr section))
(len (elf-section-size section))
(bytes (linker-object-bv o))
(relocs (linker-object-relocs o)))
(unless (= offset (elf-section-addr section))
(error "offset != addr" section))
(if (not (= (elf-section-type section) SHT_NOBITS))
(begin
(if (not (= len (bytevector-length bytes)))
(error "unexpected length" section bytes))
(bytevector-copy! bytes 0 bv offset len)
(for-each (lambda (reloc)
(process-reloc reloc bv offset addr symtab endianness))
(process-reloc reloc bv offset symtab endianness))
relocs)))))
(define (find-shstrndx objects)
"Find the section name string table in @var{objects}, and return its
section index."
(or-map (lambda (object)
(let* ((section (linker-object-section object))
(bv (linker-object-bv object))
@ -353,81 +429,177 @@
(elf-section-index section))))
objects))
;; objects ::= list of <linker-object>
;; => 3 values: ELF header, program headers, objects
(define (add-elf-objects objects endianness word-size)
"Given the list of linker objects supplied by the user, add linker
objects corresponding to parts of the ELF file: the null object, the ELF
header, and the section table.
Both of these internal objects include relocs, allowing their
inter-object references to be patched up when the final image allocation
is known. There is special support for patching up the segment table,
however. Because the segment table needs to know the segment sizes,
which is the difference between two symbols in image space, and there is
no reloc kind that is the difference between two symbols, we make a hack
and return a closure that patches up segment table entries. It seems to
work.
Returns two values: the procedure to patch the segment table, and the
list of objects, augmented with objects for the special ELF sections."
(define phoff (elf-header-len word-size))
(define phentsize (elf-program-header-len word-size))
(define shentsize (elf-section-header-len word-size))
(define shnum (+ (length objects) 3))
(define reloc-kind
(case word-size
((4) 'abs32/1)
((8) 'abs64/1)
(else (error "bad word size" word-size))))
;; ELF requires that the first entry in the section table be of type
;; SHT_NULL.
;;
(define (make-null-section)
(make-linker-object (make-elf-section #:index 0 #:type SHT_NULL
#:flags 0 #:addralign 0)
#vu8() '() '()))
;; The ELF header and the segment table.
;;
(define (make-header phnum index shoff-label)
(let* ((header (make-elf #:byte-order endianness #:word-size word-size
#:phoff phoff #:phnum phnum #:phentsize phentsize
#:shoff 0 #:shnum shnum #:shentsize shentsize
#:shstrndx (or (find-shstrndx objects) SHN_UNDEF)))
(shoff-reloc (make-linker-reloc reloc-kind
(elf-header-shoff-offset word-size)
0
shoff-label))
(size (+ phoff (* phnum phentsize)))
(bv (make-bytevector size 0)))
(write-elf-header bv header)
;; Leave the segment table uninitialized; it will be filled in
;; later by calls to the write-segment-header! closure.
(make-linker-object (make-elf-section #:index index #:type SHT_PROGBITS
#:flags SHF_ALLOC #:size size)
bv
(list shoff-reloc)
'())))
;; The section table.
;;
(define (make-footer objects shoff-label)
(let* ((size (* shentsize shnum))
(bv (make-bytevector size 0))
(section-table (make-elf-section #:index (length objects)
#:type SHT_PROGBITS
#:flags 0
#:size size)))
(define (write-and-reloc section-label section relocs)
(let ((offset (* shentsize (elf-section-index section))))
(write-elf-section-header bv offset endianness word-size section)
(if (= (elf-section-type section) SHT_NULL)
relocs
(cons* (make-linker-reloc
reloc-kind
(+ offset (elf-section-header-addr-offset word-size))
0
section-label)
(make-linker-reloc
reloc-kind
(+ offset (elf-section-header-offset-offset word-size))
0
section-label)
relocs))))
(let ((relocs (fold-values
(lambda (object relocs)
(write-and-reloc
(linker-symbol-name
(linker-object-section-symbol object))
(linker-object-section object)
relocs))
objects
(write-and-reloc shoff-label section-table '()))))
(%make-linker-object section-table bv relocs
(list (make-linker-symbol shoff-label 0))))))
(let* ((null-section (make-null-section))
(objects (cons null-section objects))
(shoff (gensym "*section-table*"))
(header (make-header (count-segments objects) (length objects) shoff))
(objects (cons header objects))
(footer (make-footer objects shoff))
(objects (cons footer objects)))
;; The header includes the segment table, which needs offsets and
;; sizes of the segments. Normally we would use relocs to rewrite
;; these values, but there is no reloc type that would allow us to
;; compute size. Such a reloc would need to take the difference
;; between two symbols, and it's probably a bad idea architecturally
;; to create one.
;;
;; So instead we return a closure to patch up the segment table.
;; Normally we'd shy away from such destructive interfaces, but it's
;; OK as we create the header section ourselves.
;;
(define (write-segment-header! segment)
(let ((bv (linker-object-bv header))
(offset (+ phoff (* (elf-segment-index segment) phentsize))))
(write-elf-program-header bv offset endianness word-size segment)))
(values write-segment-header! objects)))
(define (allocate-elf objects page-aligned? endianness word-size)
(let* ((seglists (collate-objects-into-segments objects))
(nsegments (length seglists))
(nsections (1+ (length objects))) ;; 1+ for the first reserved entry.
(program-headers-offset (elf-header-len word-size))
(fileaddr (+ program-headers-offset
(* nsegments (elf-program-header-len word-size))))
(memaddr 0))
(let lp ((seglists seglists)
(segments '())
"Lay out @var{objects} into an ELF image, computing the size of the
file, the positions of the objects, and the global symbol table.
If @var{page-aligned?} is true, read-only and writable data are
separated so that only those writable parts of the image need be mapped
with writable permissions. This makes the resulting image larger. It
is more suitable to situations where you would write a file out to disk
and read it in with mmap. Otherwise if @var{page-aligned?} is false,
sections default to 8-byte alignment.
Returns three values: the total image size, a list of objects with
relocated headers, and the global symbol table."
(receive (write-segment-header! objects)
(add-elf-objects objects endianness word-size)
(let lp ((seglists (collate-objects-into-segments objects))
(objects '())
(phidx 0)
(fileaddr fileaddr)
(memaddr memaddr)
(addr 0)
(symtab vlist-null)
(prev-flags 0))
(match seglists
((((type . flags) objs-in ...) seglists ...)
(receive (segment objs-out symtab)
(alloc-segment phidx type flags objs-in fileaddr memaddr symtab
(receive (addr objs-out symtab)
(allocate-segment
write-segment-header!
phidx type flags objs-in addr symtab
(if (and page-aligned?
(not (= flags prev-flags)))
(not (= flags prev-flags))
;; Allow sections that are not in
;; loadable segments to share pages
;; with PF_R segments.
(not (and (not type) (= PF_R prev-flags))))
*page-size*
8))
(lp seglists
(cons segment segments)
(fold-values cons objs-out objects)
(1+ phidx)
(+ (elf-segment-offset segment) (elf-segment-filesz segment))
(if (zero? (elf-segment-memsz segment))
memaddr
(+ (elf-segment-vaddr segment)
(elf-segment-memsz segment)))
(if type (1+ phidx) phidx)
addr
symtab
flags)))
(()
(let ((section-table-offset (+ (align fileaddr word-size))))
(values
(make-elf #:byte-order endianness #:word-size word-size
#:phoff program-headers-offset #:phnum nsegments
#:shoff section-table-offset #:shnum nsections
#:shstrndx (or (find-shstrndx objects) SHN_UNDEF))
(reverse segments)
(let ((null-section (make-elf-section #:index 0 #:type SHT_NULL
#:flags 0 #:addralign 0)))
(cons (make-linker-object null-section #vu8() '() '())
(reverse objects)))
symtab)))))))
(define (write-elf header segments objects symtab)
(define (phoff n)
(+ (elf-phoff header) (* n (elf-phentsize header))))
(define (shoff n)
(+ (elf-shoff header) (* n (elf-shentsize header))))
(let ((endianness (elf-byte-order header))
(word-size (elf-word-size header))
(bv (make-bytevector (shoff (elf-shnum header)) 0)))
(write-elf-header bv header)
(for-each
(lambda (segment)
(write-elf-program-header bv (phoff (elf-segment-index segment))
endianness word-size segment))
segments)
(for-each
(lambda (object)
(let ((section (linker-object-section object)))
(write-elf-section-header bv (shoff (elf-section-index section))
endianness word-size section))
(write-linker-object bv object symtab endianness))
objects)
bv))
(values addr
(reverse objects)
symtab))))))
(define (check-section-numbers objects)
"Verify that taken as a whole, that all objects have distinct,
contiguous section numbers, starting from 1. (Section 0 is the null
section.)"
(let* ((nsections (1+ (length objects))) ; 1+ for initial NULL section.
(sections (make-vector nsections #f)))
(for-each (lambda (object)
@ -443,15 +615,30 @@
(vector-set! sections n object)))))
objects)))
;; Given a list of section-header/bytevector pairs, collate the sections
;; into segments, allocate the segments, allocate the ELF bytevector,
;; and write the segments into the bytevector, relocating as we go.
;; Given a list of linker objects, collate the objects into segments,
;; allocate the segments, allocate the ELF bytevector, and write the
;; segments into the bytevector, relocating as we go.
;;
(define* (link-elf objects #:key
(page-aligned? #t)
(endianness (target-endianness))
(word-size (target-word-size)))
"Create an ELF image from the linker objects, @var{objects}.
If @var{page-aligned?} is true, read-only and writable data are
separated so that only those writable parts of the image need be mapped
with writable permissions. This is suitable for situations where you
would write a file out to disk and read it in with @code{mmap}.
Otherwise if @var{page-aligned?} is false, sections default to 8-byte
alignment.
Returns a bytevector."
(check-section-numbers objects)
(receive (header segments objects symtab)
(receive (size objects symtab)
(allocate-elf objects page-aligned? endianness word-size)
(write-elf header segments objects symtab)))
(let ((bv (make-bytevector size 0)))
(for-each
(lambda (object)
(write-linker-object bv object symtab endianness))
objects)
bv)))

5
test-suite/tests/linker.test
View file

@ -75,8 +75,9 @@
(set! elf (parse-elf bytes))
(elf? elf)))
;; 3 sections: the initial NULL section, .foo, and .shstrtab.
(pass-if-equal 3 (elf-shnum elf))
;; 5 sections: the initial NULL section, .foo, .shstrtab, the initial
;; header with segment table, and the section table.
(pass-if-equal 5 (elf-shnum elf))
(pass-if ".foo section checks out"
(let ((sec (assoc-ref (elf-sections-by-name elf) ".foo")))