From 15e97c9553f1d7bae94bff8d65285224c75fa468 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Vivek=20Das=C2=A0Mohapatra?= Date: Thu, 27 Aug 2020 17:39:17 +0100 Subject: [PATCH 1/2] Document GNU extensions to ELF related to loading and linking Program header related: PT_GNU_EH_FRAME PT_SUNW_EH_FRAME PT_GNU_STACK PT_GNU_RELRO PT_GNU_PROPERTY GNU_PROPERTY_STACK_SIZE GNU_PROPERTY_NO_COPY_ON_PROTECTED GNU_PROPERTY_LOPROC GNU_PROPERTY_HIPROC GNU_PROPERTY_LOUSER GNU_PROPERTY_HIUSER PT_DYNAMIC entries DT_GNU_FLAGS_1 (DF_GNU_1_UNIQUE) DT_GNU_PRELINKED DT_GNU_CONFLICTSZ DT_GNU_LIBLISTSZ DT_GNU_HASH DT_GNU_CONFLICT DT_GNU_LIBLIST Section header related SHT_GNU_INCREMENTAL_INPUTS SHT_GNU_ATTRIBUTES SHT_GNU_HASH SHT_GNU_LIBLIST SHT_GNU_verdef SHT_GNU_verneed SHT_GNU_versym SHT_NOTE extensions NT_GNU_ABI_TAG GNU_ABI_TAG_LINUX GNU_ABI_TAG_HURD GNU_ABI_TAG_SOLARIS GNU_ABI_TAG_FREEBSD GNU_ABI_TAG_NETBSD GNU_ABI_TAG_SYLLABLE GNU_ABI_TAG_NACL NT_GNU_HWCAP NT_GNU_BUILD_ID NT_GNU_GOLD_VERSION NT_GNU_PROPERTY_TYPE_0 --- program-loading-and-dynamic-linking.txt | 638 ++++++++++++++++++++++++ 1 file changed, 638 insertions(+) create mode 100644 program-loading-and-dynamic-linking.txt diff --git a/program-loading-and-dynamic-linking.txt b/program-loading-and-dynamic-linking.txt new file mode 100644 index 0000000..a07c195 --- /dev/null +++ b/program-loading-and-dynamic-linking.txt @@ -0,0 +1,638 @@ +Program Headers +=============== + +These are GNU extended program header type values: They are typically +found in ElfW(Phdr).p_type. + +PT_GNU_EH_FRAME 0x6474e550 +PT_SUNW_EH_FRAME 0x6474e550 + + Segment contains the EH_FRAME_HDR section (stack frame unwind information) + + NOTE: The virtual address range referred to by PT_GNU_EH_FRAME must be + covered by a PT_LOAD entry - PT_GNU_EH_FRAME on its own does not trigger + the mapping/loading of any data. + + PT_SUNW_EH_FRAME is used by a non-GNU implementation for the same purpose, + and has the same value (although this does not imply compatible contents). + + The contents of the EH_FRAME_HDR are described in the LSB. As of v5.0: + + Reference: https://refspecs.linuxfoundation.org/LSB_5.0.0/LSB-Core-generic/LSB-Core-generic/ehframechpt.html + Reference: https://refspecs.linuxfoundation.org/LSB_5.0.0/LSB-Core-generic/LSB-Core-generic/ehframechpt.html#EHFRAMEHDR + Reference: https://refspecs.linuxfoundation.org/LSB_5.0.0/LSB-Core-generic/LSB-Core-generic.html#PROGHEADER + +PT_GNU_STACK 0x6474e551 + + The p_flags member of this ElfW(Phdr) structure apply to the stack. + + If present AND p_flags DOES NOT contain PF_X (0x1) then the stack + should _not_ be executable. + + Otherwise the stack follows the architecture specific default for + executability: For example on x86 the stack is executable by default. + + NOTE: Some implementations may use this header's p_size to set the stack size. + glibc does NOT do this: See GNU_PROPERTY_STACK_SIZE instead. + + Reference: https://refspecs.linuxfoundation.org/LSB_5.0.0/LSB-Core-generic/LSB-Core-generic.html#PROGHEADER + +PT_GNU_RELRO 0x6474e552 + + The specified segment should be made read-only once run-time linking + of this object has completed. + + As with PT_GNU_EH_FRAME this header entry does NOT guarantee that the + range in question is loaded: That must be ensured via a PT_LOAD entry + which covers the range. + + Reference: https://refspecs.linuxfoundation.org/LSB_5.0.0/LSB-Core-generic/LSB-Core-generic.html#PROGHEADER + +PT_GNU_PROPERTY 0x6474e553 + + The Linux kernel uses this program header to locate the + ".note.gnu.property section". + + If there is a program property that requires the kernel to perform + some action before loading and ELF file (eg AArch64 BTI or Intel CET) + then this header MUST be present. + + If no such features are to be enabled this header MUST NOT be present. + + The contents are laid out as follows: + + Field | Length | Contents + n_namsz | 4 | 4 + n_descsz | 4 | Size of n_desc (4 byte int, processor format) + n_type | 4 | NT_GNU_PROPERTY_TYPE_0 (0x5) + n_name | 4 | GNU\0 + n_desc | n_descsz | property array + + Each element of n_desc, in turn is: + + typedef struct { + Elf_Word pr_type; + Elf_Word pr_datasz; + unsigned char pr_data[PR_DATASZ]; + unsigned char pr_padding[PR_PADDING]; + } Elf_Prop; + + pr_data is aligned to 4 bytes in 32-bit objects and 8 bytes in 64-bit ones. + + The segment itself is aligned according to the program header's p_align + field. + + PR_PADDING bytes are added _after_ PR_DATASZ so that each property is + aligned to 4 bytes (on 32 bit architectures) and to 8 bytes on 64 bit + architectures. + + This is true even if pr_datasz is 0 (cf GNU_PROPERTY_NO_COPY_ON_PROTECTED). + + Properties are sorted in ascending order of pr_type; + + Defined properties are: + + GNU_PROPERTY_STACK_SIZE 0x1 + + pr_data holds a native sized (4 bytes on 32 bit architecures, + 8 bytes on 64 bit) integer in the target processor's native format. + + The linker should pick the highest instance of this from all relocatable + objects in the link chain and ensure the stack is at least this big. + + There is no implication or requirement that the linker should or will + reduce the stack size to match this value. + + GNU_PROPERTY_NO_COPY_ON_PROTECTED 0x2 + + The linker should treat protected data symbol as defined locally at + run-time and copy this property to the output share object. + + The linker should add this property to the output share object if + any protected symbol is expected to be defined locally at run-time. + + The run-time loader should disallow copy relocations against protected + data symbols defined such objects. + + This type is expected to have a pr_datasz field of 0, and no pr_data + contents (only padding). + + GNU_PROPERTY_LOPROC 0xc0000000 + GNU_PROPERTY_HIPROC 0xdfffffff + + Reserved for processor-specific values. + + GNU_PROPERTY_LOUSER 0xe0000000 + GNU_PROPERTY_HIUSER 0xffffffff + + Reserved for application specific values. + + Reference: https://raw.githubusercontent.com/wiki/hjl-tools/linux-abi/linux-abi-draft.pdf + +There are further extensions to p_type but currently they are all +architecture specific and should be documented in the relevant ABIs. + +Dynamic segment extensions (PT_DYNAMIC entries) +=============================================== + +The following types within PT_DYNAMIC are GNU extensions: +The values are typically found in the ElfW(Dyn).d_tag member. + +DT_GNU_FLAGS_1 0x6ffffdf4 + + Similar to DT_FLAGS and DT_FLAGS_1, but DT_FLAGS is generic and + the DT_FLAGS_1 bit mask has been exhausted (last available bit + claimed by Solaris). + + Currently supports the following flag bit(s) in its d_val value: + + DF_GNU_1_UNIQUE + + This flag bit indicates that the library should be loaded at most once + across all namespaces unless a standalone copy is explicitly requested. + + Some background: + + By default libraries and all their dependencies are loaded into a single + namespace or link-map (LM_ID_BASE) - this applies to libraries loaded by + ld.so when a program starts, and to those loaded later by dlopen(3). + + glibc implements a dynamic loading extension - dlmopen(3) which is similar + to dlopen(3) but can load libraries into secondary namespaces, each of + which has its own private link map. + + Libraries in these namespaces are NOT used by the linker to resolve + symbols for one another: A library in namespaces 2 (for example) + will not use symbols or libraries from any other namespace, nor will it + be used to satisfy symbol lookups from libraries in those namespaces. + + This mechanism is the basis for isolation of LD_AUDIT libraries (for example). + + While this is generally desirable some libraries do not behave well + under these conditions - in particular libc (malloc/free get upset + when they interact with independent copies of themselves since they + have no knowledge of one another's memory accounting) and libpthread + (which tends to deadlock of two different namespaces attempt to + initialise thread metadata). + + DF_GNU_1_UNIQUE is used to mark such libraries so that when they are + loaded only one copy (which resides in LM_ID_BASE) is mapped, and + all namespaces use that copy (unless such sharing is explicitly + suppressed, such as for LD_AUDIT libraries). + + This behaviour can be explicitly overridden by the caller of dlmopen(3). + + Reference: This document is canonical. + +Prelinking +========== + +DT_GNU_PRELINKED 0x6ffffdf5 + + The d_val field contains a time_t value giving the UTC time at which the + object was (pre)linked. + + Reference: See the accompanying prelink document for details. + +DT_GNU_CONFLICTSZ 0x6ffffdf6 + + Used in prelinked objects. + d_val contains the size of the conflict segment. + +DT_GNU_LIBLISTSZ 0x6ffffdf7 + + Used in prelinked objects. + d_val contains the size of the library list. + +DT_GNU_CONFLICT 0x6ffffef8 + + Used in prelinked objects. + The d_ptr value gives the location of the conflict segment. + This will contain an array of ElfW(Rela) structs. + + If DT_GNU_LIBLIST matches the library searchlist after loading + then these relocation records are replayed immediately after + run-time loading. + +DT_GNU_LIBLIST 0x6ffffef9 + + Used in prelinked objects. + The d_ptr value gives the location of the ElfW(Lib) array giving the + SONAME, checksum and timestamp or each library encountered at prelink time. + + This is used to check that all required prelinked libraries are still + present, loaded, and have the correct checksums at runtime. + + typedef struct { + ElfW(Word) l_name; /* Name (string table index) */ + ElfW(Word) l_time_stamp; /* Timestamp */ + ElfW(Word) l_checksum; /* Checksum */ + ElfW(Word) l_version; /* Interface version */ + ElfW(Word) l_flags; /* Flags */ + } ElfW(Lib); + +Hashes +====== + +DT_GNU_HASH 0x6ffffef5 + + The d_ptr value gives the location of the GNU style symbol hash table. + + The GNU hash of a symbol is computed as follows: + - take the NAME of the symbol (WITHOUT any @version suffix) + - unsigned long h ← 5381 + - for each unsigned character C in NAME, starting at position 0: + - h ← (h << 5) + h + C; + OR + h ← (h * 33) + C; + - uint32_t HASH ← h + + Hash Table contents: + + bitmask-bits is a power of 2. + It is at least 32 (on 32 bit); at least 64 on 64 bit architectures. + There are other restrictions, see elflink.c in the binutils-gdb/bfd source. + + The bucket in which a symbol's hash entry is found is: + + gnu-hash( symbol-name ) % nbuckets + + The table is divided into 4 parts: + ---------------------------------------------------------------------------- + Part 1 (metadata): + + - nbuckets : 4 byte native integer. Number of buckets + A bucket occupies 32 bits. + + - symoffset : 4 byte native integer. + Starting index of first "real" symbol in the ".dynsym" + section, See below. + + - bitmask-words: 4 byte native integer. + The number of ELFCLASS words in part 2 of the table. + On 64-bit architctures: bitmask-bits / 64 + And on 32-bit ones : bitmask-bits / 32 + + - bloom-shift : 4 byte native integer. + The shift-count used in the bloom filter. + + symoffset: + There are synthetic symbols - one for each section in the linker output. + symoffset gives the number of such synthetic symbols ( which cannot be + looked up via the GNU hash section described here ). + + NB: symbols that _can_ be looked up via the GNU hash must be stored in + the ".dynsym" section in ascending order of bucket. + That is the ordering is determined by: + + gnu-hash( symbol-name ) % nbuckets + + ---------------------------------------------------------------------------- + Part 2 (the bloom filter bitmask): + + - bloom : ElfW(Addr)[ bitmask-words ] + + For each symbol [name] S the following is carried out (by the link editor): + - C ← __ELF_NATIVE_CLASS /* ie 32 on ELF32, 64 on ELF64 */ + - H ← gnu-hash( S ) + - BWORD ← (H / C) & (bitmask-words - 1) + - in bloom[ BWORD ] set: + - bit H & (C - 1) + - bit (H >> bloom-shift) & (C - 1) + + NOTE: The discussions and examples of this that are around may + use modulo operations instead of the logical-ands you see above: + This is not an error or divergence since: + + x % 2ⁿ ≡ x & (2ⁿ - 1) /* NB: where x is unsigned */ + + NOTE: For those unfamiliar with bloom filters: If either bit + described above is NOT SET then the hash is DEFINITELY NOT + present in the table and lookup need proceed no further. + + ---------------------------------------------------------------------------- + Part 3 (the bucket metadata): + + - bucket[nbuckets] : Array of 4 byte native integers, giving: + + For each bucket: + - The INDEX of the first symbol in that bucket, + OR 0 if no symbols in that bucket. + + NOTE: these indices give the offset into the ".dynsym" section. + For the offset into the bucket data in part 4 of the table, see below: + ---------------------------------------------------------------------------- + Part 4 (the chains, or actual bucket data): + + - chain : Contiguous arrays of pseudo hash values combining the + hash values and the index of the related symbol + Each pseudo hash value is a 4 byte native integer. + + ElfW(Word)[number-of-symbols]. + + For each symbol [name] S: + + - CHASH ← gnu-hash( S ) + - BUCKET ← CHASH % nbuckets + - CINDEX ← position of the symbol _within_ its bucket + 0 for the first symbol, 1 for the second and so forth + + The chain data are stored as a single linear chunk with each + pseudo-hash value immediately following another. CINDEX gives the + position of a pseudo-hash inside the bucket to which it belongs, + rather than its position in the chain data area as a whole. + + [ b0h0 | b0h1 | b0h3 | b1h0 | … + + - if a pseudo-hash value is the last one in the bucket: + - CHASH ← CHASH | 1 /* set the least bit */ + - else + - CHASH ← CHASH & ~1 /* unset the least bit */ + - BYTE-OFFSET ← (bucket[BUCKET] + CINDEX - symoffset) * 4 + - CHAIN-ADDR ← ((char *)&bucket[nbuckets]) + BYTE-OFFSET + - *(ElfW(Word) *)(CHAIN-ADDR) ← CHASH + + The least bit of a pseudo-hash value being set indicates that + this entry is the last in the chain - this is used during lookupg + since unlike the stock ELF hash the GNU hash does not use linked + lists to store its chains. + + Reference: https://sourceware.org/legacy-ml/binutils/2006-10/msg00377.html + Reference: https://flapenguin.me/elf-dt-gnu-hash + Reference: https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=bfd/elflink.c;h=1384c1a46b83b55876b6a73dbcba0386a458063b;hb=HEAD#l7263 + bfd_elf_size_dynsym_hash_dynstr + Reference: https://sourceware.org/git/?p=glibc.git;a=blob;f=elf/dl-lookup.c;h=807f3ea9b67489b3116535b7c433c774a72e4c29;hb=HEAD#l357 + do_lookup_x + +Section Headers +=============== + +The GNU extensions to section header type values. +Typically found in ElfW(Shdr).sh_type. + +SHT_GNU_INCREMENTAL_INPUTS 0x6fff4700 + + Section name: ".gnu_incremental_inputs" + + Currently used internally during incremental linking by gold. + +SHT_GNU_ATTRIBUTES 0x6ffffff5 + + Section name: ".gnu.attributes" + + GNU specific program attributes, sh_size bytes at sh_offset into the file. + + The first byte is the version of the attribute spec: Currently only 'A' + is defined. Each attribute is stored as: + + - len: 4 byte integer in native format (total attribute length) + - data: (len - 4) bytes of attribute data + - starting with a \0 terminated name + - at least 6 bytes of tag-byte+value + - a tag byte + - a 4 byte native integer size (including the tag byte & the size itself) + - if the tag is 2 or 3: a LEB128 encoded value stored in the remaining space + - DOCUMENTME: some attribute bytes? reverse engineer from readelf? + +SHT_GNU_HASH 0x6ffffff6 + + Section name: ".gnu.hash" (architecture specific ABI may override this) + + This section contains the GNU style hash table. See DT_GNU_HASH. + Currently only the MIPS architecture is known to use a different name. + +SHT_GNU_LIBLIST 0x6ffffff7 + + See DT_GNU_LIBLIST. + + Section name: ".gnu.liblist" + + This section should refer to a SHT_STRTAB type section via its sh_link + field: That strtab holds the names of the libraries listed in each + ElfW(Lib) struct contained in this GNU_LIBLIST section. + +Symbol Versioning +================= + +These sections implement GNU symbol versioning. +These sections all have the SHF_ALLOC atribute. + +Reference: https://refspecs.linuxfoundation.org/LSB_5.0.0/LSB-Core-generic/LSB-Core-generic.html +Reference: https://refspecs.linuxfoundation.org/LSB_5.0.0/LSB-Core-generic/LSB-Core-generic.html#SYMVERSION + +SHT_GNU_verdef 0x6ffffffd + + Section name: ".gnu.version_d" + + This section contains symbol version definitions. + + The number of entries it contains is given by the DT_VERDEFNUM entry + of the Dynamic Section SHT_DYNAMIC/".dynamic". + + The sh_link member of this section header (see the System V ABI) points to + the SHT_STRTAB section that contains the strings referenced by this section. + + This section contains an array of ElfW(Verdef) structures optionally + followed by an array of ElfW(Verdaux) structures. + + ElfW(Verdef): + + typedef struct { + ElfW(Half) vd_version; /* 0, 1 or 2. See below */ + ElfW(Half) vd_flags; /* A flag bitfield. See below */ + ElfW(Half) vd_ndx; /* Referred to by SHT_GNU_versym. See below */ + ElfW(Half) vd_cnt; /* Number of associated ElfW(Verdaux) entries */ + ElfW(Word) vd_hash; /* Version name hash (per ELF Hash function) */ + ElfW(Word) vd_aux; /* Offset in bytes from this ElfW(Verdef) + to its ElfW(Verdaux) array */ + ElfW(Word) vd_next; /* Offset in bytes frm this ElfW(Verdef) + to the next ElfW(Verdef) entry. + 0 for last entry. */ + } ElfW(Verdef); + + vd_version: + VER_DEF_NONE 0 // No version + VER_DEF_CURRENT 1 // Currrent version + + vd_flags: + VER_FLG_BASE 0x1 // [Default] Version of the whole object + VER_FLG_WEAK 0x2 // Weak version identifier + + vd_ndx: + VER_NDX_LOCAL 0 // private symbol + VER_NDX_GLOBAL 1 // global symbol + VER_NDX_LORESERVE 0xff00 // Beginning of reserved entries + VER_NDX_ELIMINATE 0xff01 // + + DOCUMENTME: VER_NDX_ELIMINATE does not appear to be implemented + in glibc: If an implementation exists its semantics should be + reverse-engineered from there and explained here. + + ElfW(Verdaux): + + typedef struct { + ElfW(Word) vda_name; // byte offset into the strtab of the version name + ElfW(Word) vda_next; // byte offset from this ElfW(Verdaux) to the next + } ElfW(Verdaux); + +SHT_GNU_verneed 0x6ffffffe + + Section name: ".gnu.version_r" + + This section contains symbol version requirements. + + The number of entries it contains is given by the DT_VERNEEDNUM entry + of the Dynamic Section SHT_DYNAMIC/".dynamic". + + The sh_link member of this section header (see the System V ABI) points to + the SHT_STRTAB section that contains the strings referenced by this section. + + This section contains an array of ElfW(Verneed) structures optionally + followed by an array of ElfW(Vernaux) structures. + + ElfW(Verneed): + + typedef struct { + ElfW(Half) vn_version; /* See below */ + ElfW(Half) vn_cnt; /* Number of associated ElfW(Vernaux) entries */ + ElfW(Word) vn_file; /* Byte offset in strtab of required DSO filename */ + ElfW(Word) vn_aux; /* Byte offset from this ElfW(Verneed) + to its ElfW(Vernaux) array */ + ElfW(Word) vn_next; /* Byte offset from this ElfW(Verneed) to the + next one. 0 in the last one */ + } ElfW(Verneed); + + ElfW(Vernaux): + + typedef struct { + ElfW(Word) vna_hash; /* Dependency name hash (per ELF hash function) */ + ElfW(Half) vna_flags; /* Dependency flag bitfield. See below */ + ElfW(Half) vna_other; /* Referred to by SHT_GNU_versym, but see below */ + ElfW(Word) vna_name; /* Byte offset in strtab of required (symbol) name */ + ElfW(Word) vna_next; /* Byte offset from this ElfW(Vernaux) to the next + 0 for the last entry */ + } ElfW(Vernaux); + + vna_flags: + VER_FLG_WEAK 0x2 Weak version identifier: + Not fatal if this symbol+version is missing. + + vna_other: + This value is used to look up the symbol version hash: It gives + the position of the hash in the version lookup table. + + Bit 15 (0x8000) is a flag bit and should be masked out of + this value before using it as an index (eg by bitwise-and-ing + its value with 0x7fff) + + If bit 15 (0x8000) is set then this symbol is hidden and is + never an acceptable candidate for matching version criteria. + + Reference: glibc: elf/dl-version.c; elf/dl-lookup.c + +SHT_GNU_versym 0x6fffffff + + Section name: ".gnu.version" + + The versioned symbol table. + + If present, this must have the same number ofentries as the + SHT_DYNSYM/".dynsym" section. The entries in this section are in the + same order as those in SHT_DYNSYM. + + That is to say: Entry 2 in this table corresponds to entry 2 in + SHT_DYNSYM, entry 3 here to entry 3 in SHT_DYNSYM, and so on. + + This section contains an array of elements of type ElfW(Half). + + Each entry specifies the version defined for or required by the + corresponding symbol in the Dynamic Symbol Table. + + Entries do not give the version directly - instead they refer to the + corresponding ElfW(Vernaux).vna_other or ElfW(Verdef).vd_ndx structure+member. + +Two values are reserved: + + VER_NDX_LOCAL 0 - The symbol is private, and is not available outside this object. + VER_NDX_GLOBAL 1 - The symbol is globally available (ie the base or default version). + +Note section descriptors (SHT_NOTE extensions) +============================================== + +These SHT_NOTE descriptor types are GNU extensions +Found in the type field of the ELF note layout. + +Section name: ".note" as per standard SHT_NOTE sections. + +Each note entry should be aligned to 4 bytes in 32-bit objects or +8 bytes in 64-bit objects (see below for exceptions to this). + +Alignment: A note parser should use p_align from the program section +header for note alignment rather than assuming alignment based on ELF +file class. + +NT_GNU_ABI_TAG 1 + + Use to indicate kernel type and minimum kernel version: + Section must be named ".note.ABI-tag" + + Alignment: Always 4-bytes, Even on 64 bit architectures. + + The name field (namesz/name) contains the string "GNU". + + The descsz field must be at least 16, + The first 16 bytes of the desc [aka descdata] field are as follows: + + The first 4 byte word is a native integer indicating the kernel type: + GNU_ABI_TAG_LINUX 0 + GNU_ABI_TAG_HURD 1 + GNU_ABI_TAG_SOLARIS 2 + GNU_ABI_TAG_FREEBSD 3 + GNU_ABI_TAG_NETBSD 4 + GNU_ABI_TAG_SYLLABLE 5 + GNU_ABI_TAG_NACL 6 + + The second, third, and fourth 4-byte words of the desc field contain the earliest + compatible kernel version. + + For example, if the 3 words are 2, 2, and 5, this signifies a 2.2.5 kernel. + +NT_GNU_HWCAP 2 + + The first 4 bytes are a native integer giving the number of capabilities. + + The next 4 bytes give the bitmask of enabled capabilities. + + The remainder is a packed array of: + [ 1 byte ][ N bytes ] + [ TESTBIT ][ \0 terminated cap name ] + +NT_GNU_BUILD_ID 3 + + descsz bytes of build-id data. + + Alignment: Always 4-bytes, Even on 64 bit architectures. + + Typically presented as a hex string by user-facing tools. + Stored as binary (ie not necessarily printable, not encoded). + + The build-id is desctribed as having the following properties: + It is unique among the set of meaningful contents for ELF files and + identical when the output filewould otherwise have been identical. + + The computation mechanism for the build-id is not given, and is in + any case opaque after compile time. + +NT_GNU_GOLD_VERSION 4 + + Up to descsz of [printable] gold version string bytes. + +NT_GNU_PROPERTY_TYPE_0 5 + + 32 or 64 bit aligned (matching the architecture) bytes of data. + Each entry within this data blob consists of: + + 4 bytes, a native integer giving the subtype. + 4 bytes, a native integer giving the size of the entry + + See PT_GNU_PROPERTY and/or architecture specific ABIs for details. -- 2.20.1