Merging r369828:

------------------------------------------------------------------------
r369828 | maskray | 2019-08-24 02:41:15 +0200 (Sat, 24 Aug 2019) | 18 lines

[ELF] Align the first section of a PT_LOAD even if its type is SHT_NOBITS

Reported at https://reviews.llvm.org/D64930#1642223

If the only section of a PT_LOAD is a SHT_NOBITS section (e.g. .bss), we
may not align its sh_offset. p_offset of the PT_LOAD will be set to
sh_offset, and we will get p_offset!=p_vaddr (mod p_align).  If such
executable is mapped by the Linux kernel, it will segfault.

After D64906, this may happen the non-linker script case.

The linker script case has had this issue for a long time.
This was fixed by rL321657 (but the test linkerscript/nobits-offset.s
failed to test a SHT_NOBITS section), but broken by rL345154.

Reviewed By: peter.smith

Differential Revision: https://reviews.llvm.org/D66658
------------------------------------------------------------------------

llvm-svn: 371196

lld/ELF/Writer.cpp

@@ -2230,25 +2230,26 @@ template <class ELFT> void Writer<ELFT>::fixSectionAlignments() {
 // same with its virtual address modulo the page size, so that the loader can
 // load executables without any address adjustment.
 static uint64_t computeFileOffset(OutputSection *os, uint64_t off) {
-  // File offsets are not significant for .bss sections. By convention, we keep
-  // section offsets monotonically increasing rather than setting to zero.
-  if (os->type == SHT_NOBITS)
-    return off;
-
-  // If the section is not in a PT_LOAD, we just have to align it.
-  if (!os->ptLoad)
-    return alignTo(off, os->alignment);
-
   // The first section in a PT_LOAD has to have congruent offset and address
   // module the page size.
-  OutputSection *first = os->ptLoad->firstSec;
-  if (os == first) {
+  if (os->ptLoad && os->ptLoad->firstSec == os) {
     uint64_t alignment = std::max<uint64_t>(os->alignment, config->maxPageSize);
     return alignTo(off, alignment, os->addr);
   }
 
+  // File offsets are not significant for .bss sections other than the first one
+  // in a PT_LOAD. By convention, we keep section offsets monotonically
+  // increasing rather than setting to zero.
+   if (os->type == SHT_NOBITS)
+     return off;
+
+  // If the section is not in a PT_LOAD, we just have to align it.
+  if (!os->ptLoad)
+    return alignTo(off, os->alignment);
+
   // If two sections share the same PT_LOAD the file offset is calculated
   // using this formula: Off2 = Off1 + (VA2 - VA1).
+  OutputSection *first = os->ptLoad->firstSec;
   return first->offset + os->addr - first->addr;
 }
 

lld/test/ELF/basic-ppc64.s

@@ -35,7 +35,7 @@
 // CHECK-NEXT:  Version: 1
 // CHECK-NEXT:  Entry: 0x10000
 // CHECK-NEXT:  ProgramHeaderOffset: 0x40
-// CHECK-NEXT:  SectionHeaderOffset: 0x200F8
+// CHECK-NEXT:  SectionHeaderOffset: 0x30098
 // CHECK-NEXT:  Flags [ (0x2)
 // CHECK-NEXT:    0x2
 // CHECK-NEXT:  ]
@@ -178,7 +178,7 @@
 // CHECK-NEXT:      SHF_WRITE (0x1)
 // CHECK-NEXT:    ]
 // CHECK-NEXT:    Address: 0x30000
-// CHECK-NEXT:    Offset: 0x20060
+// CHECK-NEXT:    Offset: 0x30000
 // CHECK-NEXT:    Size: 0
 // CHECK-NEXT:    Link: 0
 // CHECK-NEXT:    Info: 0
@@ -194,7 +194,7 @@
 // CHECK-NEXT:      SHF_STRINGS (0x20)
 // CHECK-NEXT:    ]
 // CHECK-NEXT:    Address: 0x0
-// CHECK-NEXT:    Offset: 0x20060
+// CHECK-NEXT:    Offset: 0x30000
 // CHECK-NEXT:    Size: 8
 // CHECK-NEXT:    Link: 0
 // CHECK-NEXT:    Info: 0
@@ -211,7 +211,7 @@
 // CHECK-NEXT:    Flags [ (0x0)
 // CHECK-NEXT:    ]
 // CHECK-NEXT:    Address: 0x0
-// CHECK-NEXT:    Offset: 0x20068
+// CHECK-NEXT:    Offset: 0x30008
 // CHECK-NEXT:    Size: 48
 // CHECK-NEXT:    Link: 10
 // CHECK-NEXT:    Info: 2
@@ -233,7 +233,7 @@
 // CHECK-NEXT:    Flags [ (0x0)
 // CHECK-NEXT:    ]
 // CHECK-NEXT:    Address: 0x0
-// CHECK-NEXT:    Offset: 0x20098
+// CHECK-NEXT:    Offset: 0x30038
 // CHECK-NEXT:    Size: 84
 // CHECK-NEXT:    Link: 0
 // CHECK-NEXT:    Info: 0
@@ -255,7 +255,7 @@
 // CHECK-NEXT:    Flags [ (0x0)
 // CHECK-NEXT:    ]
 // CHECK-NEXT:    Address: 0x0
-// CHECK-NEXT:    Offset: 0x200EC
+// CHECK-NEXT:    Offset: 0x3008C
 // CHECK-NEXT:    Size: 10
 // CHECK-NEXT:    Link: 0
 // CHECK-NEXT:    Info: 0

lld/test/ELF/linkerscript/nobits-offset.s

@@ -2,17 +2,24 @@
 # RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
 # RUN: echo "SECTIONS { \
 # RUN:         .sec1 (NOLOAD) : { . += 1; } \
-# RUN:         .text          : { *(.text) } \
+# RUN:         .bss           : { *(.bss) } \
 # RUN:       };" > %t.script
 # RUN: ld.lld %t.o -T %t.script -o %t
-# RUN: llvm-readelf --sections %t | FileCheck %s
+# RUN: llvm-readelf -S -l %t | FileCheck %s
 
-# We used to misalign section offsets if the first section in a
-# PT_LOAD was SHT_NOBITS.
+## If a SHT_NOBITS section is the only section of a PT_LOAD segment,
+## p_offset will be set to the sh_offset field of the section. Check we align
+## sh_offset to sh_addr modulo max-page-size, so that p_vaddr=p_offset (mod
+## p_align).
 
-# CHECK: [ 2] .text  PROGBITS  0000000000000010 001010 000010 00  AX  0   0 16
+# CHECK: Name Type   Address          Off     Size   ES Flg Lk Inf Al
+# CHECK: .bss NOBITS 0000000000000400 001400  000001 00  WA  0   0 1024
 
-.global _start
-_start:
-  nop
-.p2align 4
+# CHECK: Type Offset   VirtAddr           PhysAddr           FileSiz  MemSiz   Flg Align
+# CHECK: LOAD 0x001400 0x0000000000000400 0x0000000000000400 0x000000 0x000001 RW  0x1000
+
+# CHECK: 00 .bss
+
+.bss
+.p2align 10
+.byte 0

lld/test/ELF/nobits-offset.s

@@ -0,0 +1,21 @@
+# REQUIRES: aarch64
+# RUN: llvm-mc -filetype=obj -triple=aarch64 %s -o %t.o
+# RUN: ld.lld %t.o -o %t
+# RUN: llvm-readelf -S -l %t | FileCheck %s
+
+## If a SHT_NOBITS section is the only section of a PT_LOAD segment,
+## p_offset will be set to the sh_offset field of the section. Check we align
+## sh_offset to sh_addr modulo max-page-size, so that p_vaddr=p_offset (mod
+## p_align).
+
+# CHECK: Name Type   Address          Off     Size   ES Flg Lk Inf Al
+# CHECK: .bss NOBITS 0000000000210000 010000  000001 00  WA  0   0 4096
+
+# CHECK: Type Offset   VirtAddr           PhysAddr           FileSiz  MemSiz   Flg Align
+# CHECK: LOAD 0x010000 0x0000000000210000 0x0000000000210000 0x000000 0x000001 RW  0x10000
+
+# CHECK: 02 .bss
+
+.bss
+.p2align 12
+.byte 0

lld/test/ELF/relocation-copy-align-common.s

@@ -15,7 +15,7 @@
 # CHECK-NEXT:     SHF_WRITE
 # CHECK-NEXT:   ]
 # CHECK-NEXT:   Address: 0x203000
-# CHECK-NEXT:   Offset: 0x20B0
+# CHECK-NEXT:   Offset: 0x3000
 # CHECK-NEXT:   Size: 16
 # CHECK-NEXT:   Link: 0
 # CHECK-NEXT:   Info: 0