[yaml2obj/obj2yaml/llvm-readobj] Support SHF_GNU_RETAIN

In binutils, the flag is defined for ELFOSABI_GNU and ELFOSABI_FREEBSD.
It can be used to mark a section as a GC root.

In practice, the flag has generic semantics and can be applied to many
EI_OSABI values, so we consider it generic.

Differential Revision: https://reviews.llvm.org/D95728

include/llvm/BinaryFormat/ELF.h

@@ -949,6 +949,9 @@ enum : unsigned {
   // Identifies a section containing compressed data.
   SHF_COMPRESSED = 0x800U,
 
+  // This section should not be garbage collected by the linker.
+  SHF_GNU_RETAIN = 0x200000,
+
   // This section is excluded from the final executable or shared library.
   SHF_EXCLUDE = 0x80000000U,
 

lib/ObjectYAML/ELFYAML.cpp

@@ -555,6 +555,7 @@ void ScalarBitSetTraits<ELFYAML::ELF_SHF>::bitset(IO &IO,
   BCase(SHF_GROUP);
   BCase(SHF_TLS);
   BCase(SHF_COMPRESSED);
+  BCase(SHF_GNU_RETAIN);
   switch (Object->getMachine()) {
   case ELF::EM_ARM:
     BCase(SHF_ARM_PURECODE);

test/tools/llvm-readobj/ELF/gnu-sections.test

@@ -23,7 +23,7 @@
 # ELF32-NEXT:  W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
 # ELF32-NEXT:  L (link order), O (extra OS processing required), G (group), T (TLS),
 # ELF32-NEXT:  C (compressed), x (unknown), o (OS specific), E (exclude),
-# ELF32-NEXT:  p (processor specific)
+# ELF32-NEXT:  R (retain), p (processor specific)
 
 --- !ELF
 FileHeader:
@@ -97,7 +97,7 @@ Symbols:
 # ELF64-NEXT:  W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
 # ELF64-NEXT:  L (link order), O (extra OS processing required), G (group), T (TLS),
 # ELF64-NEXT:  C (compressed), x (unknown), o (OS specific), E (exclude),
-# ELF64-NEXT:  l (large), p (processor specific)
+# ELF64-NEXT:  R (retain), l (large), p (processor specific)
 
 ## For an EM_ARM target we print "y" for the SHF_ARM_PURECODE section flag.
 ## Check we mention it in the flag key.
@@ -109,4 +109,4 @@ Symbols:
 # ARM-NEXT:  W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
 # ARM-NEXT:  L (link order), O (extra OS processing required), G (group), T (TLS),
 # ARM-NEXT:  C (compressed), x (unknown), o (OS specific), E (exclude),
-# ARM-NEXT:  y (purecode), p (processor specific)
+# ARM-NEXT:  R (retain), y (purecode), p (processor specific)

test/tools/llvm-readobj/ELF/section-flags-os-proc.test

@@ -11,10 +11,12 @@
 # OS-PROC-LLVM:      Name: .os.flags.high
 # OS-PROC-LLVM-NEXT: Type: SHT_PROGBITS
 # OS-PROC-LLVM-NEXT: Flags [ (0xFE00000)
+# OS-PROC-LLVM-NEXT:   SHF_GNU_RETAIN (0x200000)
 # OS-PROC-LLVM-NEXT: ]
 # OS-PROC-LLVM:      Name: .os.flags.mask
 # OS-PROC-LLVM-NEXT: Type: SHT_PROGBITS
 # OS-PROC-LLVM-NEXT: Flags [ (0xFF00000)
+# OS-PROC-LLVM-NEXT:   SHF_GNU_RETAIN (0x200000)
 # OS-PROC-LLVM-NEXT: ]
 # OS-PROC-LLVM:      Name: .proc.flags.low
 # OS-PROC-LLVM-NEXT: Type: SHT_PROGBITS
@@ -38,11 +40,13 @@
 # OS-PROC-LLVM-NEXT: Type: SHT_PROGBITS
 # OS-PROC-LLVM-NEXT: Flags [ (0xEFE00000)
 # OS-PROC-LLVM-NEXT:   SHF_EXCLUDE (0x80000000)
+# OS-PROC-LLVM-NEXT:   SHF_GNU_RETAIN (0x200000)
 # OS-PROC-LLVM-NEXT: ]
 # OS-PROC-LLVM:      Name: .both.flags.mask
 # OS-PROC-LLVM-NEXT: Type: SHT_PROGBITS
 # OS-PROC-LLVM-NEXT: Flags [ (0xFFF00000)
 # OS-PROC-LLVM-NEXT:   SHF_EXCLUDE (0x80000000)
+# OS-PROC-LLVM-NEXT:   SHF_GNU_RETAIN (0x200000)
 # OS-PROC-LLVM-NEXT: ]
 # OS-PROC-LLVM:      Name: .exclude
 # OS-PROC-LLVM-NEXT: Type: SHT_PROGBITS
@@ -56,6 +60,7 @@
 # OS-PROC-LLVM-NEXT:   SHF_COMPRESSED (0x800)
 # OS-PROC-LLVM-NEXT:   SHF_EXCLUDE (0x80000000)
 # OS-PROC-LLVM-NEXT:   SHF_EXECINSTR (0x4)
+# OS-PROC-LLVM-NEXT:   SHF_GNU_RETAIN (0x200000)
 # OS-PROC-LLVM-NEXT:   SHF_GROUP (0x200)
 # OS-PROC-LLVM-NEXT:   SHF_INFO_LINK (0x40)
 # OS-PROC-LLVM-NEXT:   SHF_LINK_ORDER (0x80)
@@ -70,13 +75,13 @@
 # OS-PROC-GNU-NEXT:  [Nr] Name                   Type     Address          Off    Size   ES Flg            Lk Inf Al
 # OS-PROC-GNU-NEXT:  [ 0]                        NULL     0000000000000000 000000 000000 00                0   0  0
 # OS-PROC-GNU-NEXT:  [ 1] .os.flags.low          PROGBITS 0000000000000000 000040 000000 00   o            0   0  0
-# OS-PROC-GNU-NEXT:  [ 2] .os.flags.high         PROGBITS 0000000000000000 000040 000000 00   o            0   0  0
+# OS-PROC-GNU-NEXT:  [ 2] .os.flags.high         PROGBITS 0000000000000000 000040 000000 00  Ro            0   0  0
 # OS-PROC-GNU-NEXT:  [ 3] .os.flags.mask         PROGBITS 0000000000000000 000040 000000 00   o            0   0  0
 # OS-PROC-GNU-NEXT:  [ 4] .proc.flags.low        PROGBITS 0000000000000000 000040 000000 00   p            0   0  0
 # OS-PROC-GNU-NEXT:  [ 5] .proc.flags.high       PROGBITS 0000000000000000 000040 000000 00   p            0   0  0
 # OS-PROC-GNU-NEXT:  [ 6] .proc.flags.mask       PROGBITS 0000000000000000 000040 000000 00   p            0   0  0
 # OS-PROC-GNU-NEXT:  [ 7] .both.flags.low        PROGBITS 0000000000000000 000040 000000 00  op            0   0  0
-# OS-PROC-GNU-NEXT:  [ 8] .both.flags.high       PROGBITS 0000000000000000 000040 000000 00  op            0   0  0
+# OS-PROC-GNU-NEXT:  [ 8] .both.flags.high       PROGBITS 0000000000000000 000040 000000 00 Rop            0   0  0
 # OS-PROC-GNU-NEXT:  [ 9] .both.flags.mask       PROGBITS 0000000000000000 000040 000000 00  op            0   0  0
 # OS-PROC-GNU-NEXT:  [10] .exclude               PROGBITS 0000000000000000 000040 000000 00   E            0   0  0
 # OS-PROC-GNU-NEXT:  [11] .all.possible          PROGBITS 0000000000000000 000040 000000 00 WAXMSILOGTCopx 0   0  0

test/tools/llvm-readobj/ELF/section-flags.test

@@ -6,11 +6,12 @@
 
 # LLVM:      Name: allflags
 # LLVM-NEXT: Type: SHT_PROGBITS
-# LLVM-NEXT: Flags [ (0x80000FF7)
+# LLVM-NEXT: Flags [ (0x80200FF7)
 # LLVM-NEXT:   SHF_ALLOC (0x2)
 # LLVM-NEXT:   SHF_COMPRESSED (0x800)
 # LLVM-NEXT:   SHF_EXCLUDE (0x80000000)
 # LLVM-NEXT:   SHF_EXECINSTR (0x4)
+# LLVM-NEXT:   SHF_GNU_RETAIN (0x200000)
 # LLVM-NEXT:   SHF_GROUP (0x200)
 # LLVM-NEXT:   SHF_INFO_LINK (0x40)
 # LLVM-NEXT:   SHF_LINK_ORDER (0x80)
@@ -92,6 +93,14 @@
 # LLVM-NEXT:   SHF_COMPRESSED (0x800)
 # LLVM-NEXT: ]
 
+## SHF_GNU_RETAIN is defined in the OS-specific range, but we treat it generic
+## and allow it to be used with OSABI values other than GNU and FREEBSD.
+# LLVM:      Name: retain
+# LLVM-NEXT: Type: SHT_PROGBITS
+# LLVM-NEXT: Flags [ (0x200000)
+# LLVM-NEXT:   SHF_GNU_RETAIN (0x200000)
+# LLVM-NEXT: ]
+
 # LLVM:      Name: exclude
 # LLVM-NEXT: Type: SHT_PROGBITS
 # LLVM-NEXT: Flags [ (0x80000000)
@@ -104,7 +113,7 @@
 # LLVM-NEXT: ]
 
 # GNU: [Nr] Name          Type     Address          Off    Size   ES Flg          Lk Inf Al
-# GNU: [ 1] allflags      PROGBITS 0000000000000000 000040 000000 00 WAXMSILOGTCE 0   0  0
+# GNU: [ 1] allflags      PROGBITS 0000000000000000 000040 000000 00 WAXMSILOGTCRE 0   0  0
 # GNU: [ 2] noflags       PROGBITS 0000000000000000 000040 000000 00              0   0  0
 # GNU: [ 3] write         PROGBITS 0000000000000000 000040 000000 00 W            0   0  0
 # GNU: [ 4] alloc         PROGBITS 0000000000000000 000040 000000 00 A            0   0  0
@@ -117,8 +126,9 @@
 # GNU: [11] group         PROGBITS 0000000000000000 000040 000000 00 G            0   0  0
 # GNU: [12] tls           PROGBITS 0000000000000000 000040 000000 00 T            0   0  0
 # GNU: [13] compressed    PROGBITS 0000000000000000 000040 000000 00 C            0   0  0
-# GNU: [14] exclude       PROGBITS 0000000000000000 000040 000000 00 E            0   0  0
+# GNU: [14] retain        PROGBITS 0000000000000000 000040 000000 00 R            0   0  0
-# GNU: [15] unknown       PROGBITS 0000000000000000 000040 000000 00 x            0   0  0
+# GNU: [15] exclude       PROGBITS 0000000000000000 000040 000000 00 E            0   0  0
+# GNU: [16] unknown       PROGBITS 0000000000000000 000040 000000 00 x            0   0  0
 
 --- !ELF
 FileHeader:
@@ -131,7 +141,7 @@ Sections:
     Type:  SHT_PROGBITS
     Flags: [ SHF_WRITE, SHF_ALLOC, SHF_EXECINSTR, SHF_MERGE,
             SHF_STRINGS, SHF_INFO_LINK, SHF_LINK_ORDER, SHF_OS_NONCONFORMING,
-            SHF_GROUP, SHF_TLS, SHF_COMPRESSED, SHF_EXCLUDE ]
+            SHF_GROUP, SHF_TLS, SHF_COMPRESSED, SHF_GNU_RETAIN, SHF_EXCLUDE ]
   - Name:  noflags
     Type:  SHT_PROGBITS
     Flags: [ ]
@@ -168,6 +178,9 @@ Sections:
   - Name:  compressed
     Type:  SHT_PROGBITS
     Flags: [ SHF_COMPRESSED ]
+  - Name:  retain
+    Type:  SHT_PROGBITS
+    Flags: [ SHF_GNU_RETAIN ]
   - Name:  exclude
     Type:  SHT_PROGBITS
     Flags: [ SHF_EXCLUDE ]

tools/llvm-readobj/ELFDumper.cpp

@@ -1201,6 +1201,7 @@ static const EnumEntry<unsigned> ElfSectionFlags[] = {
   ENUM_ENT(SHF_GROUP,            "G"),
   ENUM_ENT(SHF_TLS,              "T"),
   ENUM_ENT(SHF_COMPRESSED,       "C"),
+  ENUM_ENT(SHF_GNU_RETAIN,       "R"),
   ENUM_ENT(SHF_EXCLUDE,          "E"),
 };
 
@@ -3483,15 +3484,14 @@ static void printSectionDescription(formatted_raw_ostream &OS,
   OS << "  L (link order), O (extra OS processing required), G (group), T "
         "(TLS),\n";
   OS << "  C (compressed), x (unknown), o (OS specific), E (exclude),\n";
+  OS << "  R (retain)";
 
   if (EMachine == EM_X86_64)
-    OS << "  l (large), ";
+    OS << ", l (large)";
   else if (EMachine == EM_ARM)
-    OS << "  y (purecode), ";
+    OS << ", y (purecode)";
-  else
-    OS << "  ";
 
-  OS << "p (processor specific)\n";
+  OS << ", p (processor specific)\n";
 }
 
 template <class ELFT> void GNUELFDumper<ELFT>::printSectionHeaders() {