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All about symbol versioning

In 1995, Solaris' link editor and ld.so introduced the symbol versioning mechanism. Ulrich Drepper and Eric Youngdale borrowed Solaris symbol versioning in 1997 and designed the GNU style symbol versioning for glibc.

When a shared object is updated, the behavior of a symbol changes (ABI changes (such as changing the type of parameters or return values) or behavior changes), traditionally a DT_SONAME bump is required. Otherwise a dependent application/shared object built with the old version may run abnormally. This can be inconvenient if the number of dependent applications is large.

Symbol versioning provides backward compatibility without changing DT_SONAME.

The following part describes the representation, and then describes the behaviors from the perspectives of assembler, linker, and ld.so. One may wish to skip the representation part when reading for the first time.

Representation

In a shared object or executable file that uses symbol versioning, there are up to three sections related to symbol versioning. .gnu.version_r and .gnu.version_d among them are optional:

  • .gnu.version (version symbol section). The DT_VERSYM tag in the dynamic table points to the section. Assuming there are N entries in .dynsym, .gnu.version contains N uint16_t values, with the i-th entry indicating the version ID of the i-th symbol. Put it another way, .gnu.version is a parallel table to .dynsym.
  • .gnu.version_r (version requirement section). The DT_VERNEED/ DT_VERNEEDNUM tags in the dynamic table delimiter this section. This section describes the version information used by the undefined versioned symbol in the module.
  • .gnu.version_d (version definition section). The DT_VERDEF/DT_VERDEFNUM tags in the dynamic table delimiter this section. This section describes the version information used by the defined versioned symbols in the module.
// Version definitions
typedef struct {
  Elf64_Half    vd_version;  // version: 1
  Elf64_Half    vd_flags;    // VER_FLG_BASE (index 1) or 0 (index != 1)
  Elf64_Half    vd_ndx;      // version index
  Elf64_Half    vd_cnt;      // number of associated aux entries, always 1 in practice
  Elf64_Word    vd_hash;     // SysV hash of the version name
  Elf64_Word    vd_aux;      // offset in bytes to the verdaux array
  Elf64_Word    vd_next;     // offset in bytes to the next verdef entry
} Elf64_Verdef;

typedef struct {
  Elf64_Word    vda_name;    // version name
  Elf64_Word    vda_next;    // offset in bytes to the next verdaux entry
} Elf64_Verdaux;

// Version needs
typedef struct {
  Elf64_Half    vn_version;  // version: 1
  Elf64_Half    vn_cnt;      // number of associated aux entries
  Elf64_Word    vn_file;     // .dynstr offset of the depended filename
  Elf64_Word    vn_aux;      // offset in bytes to vernaux array
  Elf64_Word    vn_next;     // offset in bytes to next verneed entry
} Elf64_Verneed;

typedef struct {
  Elf64_Word    vna_hash;    // SysV hash of vna_name
  Elf64_Half    vna_flags;   // usually 0; copied from vd_flags of the depended so
  Elf64_Half    vna_other;   // unused
  Elf64_Word    vna_name;    // .dynstr offset of the version name
  Elf64_Word    vna_next;    // offset in bytes to next vernaux entry
} Elf64_Vernaux;

Currently GNU ld does not set the VER_FLG_WEAK flag. BZ24718#c15 proposed "set VER_FLG_WEAK on version reference if all symbols are weak".

The advantage of using a parallel table for .gnu.version is that symbol versioning is optional. ld.so implementations which do not support symbol versioning can freely assume no symbol has a version. The behavior is that all references as if bind to the default version definitions. musl ld.so falls into this category.

Version index values

Index 0 is called VER_NDX_LOCAL. The binding of the symbol will be changed to STB_LOCAL. Index 1 is called VER_NDX_GLOBAL. It has no special effect and is used for unversioned symbols. Index 2 to 0xffef are used for user defined versions.

Defined versioned symbols have two forms:

  • foo@@v2, the default version.
  • foo@v2, a non-default version (hidden version). The VERSYM_HIDDEN bit of the version ID is set.

Undefined versioned symbols have only the foo@v2 form.

Usually versioned symbols are only defined in shared objects, but executables can have defined versioned symbols as well. (When a shared object is updated, the old symbols are retained so that other shared objects do not need to be relinked, and executable files usually do not provide versioned symbols for other shared objects to reference.)

Example

readelf -V can dump the symbol versioning tables.

In the .gnu.version_d output below:

  • Version index 1 (VER_NDX_GLOBAL) is the filename (soname if shared object). The VER_FLG_BASE flag is set.
  • Version index 2 is a user defined version. Its name is LUA_5.3.

In the .gnu.version_r output below, each of version indexes 3~10 represents a version in a depended shared object. The name GLIBC_2.2.5 appears thrice, each for a different shared object.

The .gnu.version table assigns a version index to each .dynsym entry.

% readelf -V /usr/bin/lua5.3

Version symbols section '.gnu.version' contains 248 entries:
 Addr: 0x0000000000002af4  Offset: 0x002af4  Link: 5 (.dynsym)
  000:   0 (*local*)       3 (GLIBC_2.3)     4 (GLIBC_2.2.5)   4 (GLIBC_2.2.5)
  004:   5 (GLIBC_2.3.4)   4 (GLIBC_2.2.5)   4 (GLIBC_2.2.5)   4 (GLIBC_2.2.5)
  ...

Version definition section '.gnu.version_d' contains 2 entries:
 Addr: 0x0000000000002ce8  Offset: 0x002ce8  Link: 6 (.dynstr)
  000000: Rev: 1  Flags: BASE  Index: 1  Cnt: 1  Name: lua5.3
  0x001c: Rev: 1  Flags: none  Index: 2  Cnt: 1  Name: LUA_5.3

Version needs section '.gnu.version_r' contains 3 entries:
 Addr: 0x0000000000002d20  Offset: 0x002d20  Link: 6 (.dynstr)
  000000: Version: 1  File: libdl.so.2  Cnt: 1
  0x0010:   Name: GLIBC_2.2.5  Flags: none  Version: 9
  0x0020: Version: 1  File: libm.so.6  Cnt: 1
  0x0030:   Name: GLIBC_2.2.5  Flags: none  Version: 6
  0x0040: Version: 1  File: libc.so.6  Cnt: 6
  0x0050:   Name: GLIBC_2.11  Flags: none  Version: 10
  0x0060:   Name: GLIBC_2.14  Flags: none  Version: 8
  0x0070:   Name: GLIBC_2.4  Flags: none  Version: 7
  0x0080:   Name: GLIBC_2.3.4  Flags: none  Version: 5
  0x0090:   Name: GLIBC_2.2.5  Flags: none  Version: 4
  0x00a0:   Name: GLIBC_2.3  Flags: none  Version: 3

Symbol versioning in object files

The GNU scheme allows .symver directives to label the versions of the symbols in objec files. The symbol names residing in .o contain @ or @@.

Assembler behavior

GNU as and LLVM integrated assembler provide implementation.

  • .symver foo, foo@v1
    • If foo is undefined, produce foo@v1
    • If foo is defined, produce foo and foo@v1 with the same binding (STB_LOCAL, STB_WEAK, or STB_GLOBAL) and st_other value (i.e. the same visibility). Personally I think this behavior is a design flaw {gas-copy}. The proposed V4 PATCH gas: Extend .symver directive can address this problem.
  • .symver foo, foo@@v1
    • If foo is undefined, error
    • If foo is defined, produce foo and foo@v1 with the same binding and st_other value.
  • .symver foo, foo@@@v1
    • If foo is undefined, produce foo@v1
    • If foo is defined, produce foo@@v1

Personal recommendation:

  • To define a default version symbol: use .symver foo, foo@@@v2 so that foo is not present.
  • To define a non-default version symbol, add a suffix to the original symbol name (.symver foo_v1, foo@v1) to prevent conflicts with foo. This will however leave (usually undesirable) foo_v1. If you don't strip foo_v1 from the object file, you may localize it with a local: pattern in the version script. With GNU as 2.35 (PR25295), you can use .symver foo_v1, foo@v1, remove
  • The version of an undefined symbol is usually bound at link time. It is usually unnecessary to set the version with .symver. If required, prefer .symver foo, foo@@@v1 to .symver foo, foo@v1.

Linker behavior

The linker enters the symbol resolution stage after reading in object files, archive files, shared objects, LTO files, linker scripts, etc.

GNU ld uses indirect symbol to represent versioned symbols. There are complicated rules, and these rules are not documented. The symbol resolution rules that I personally derived:

  • Defined foo resolves undefined foo (traditional unversioned rule)
  • Defined foo@v1 resolves undefined foo@v1 (a non-default version symbol is like a separate symbol)
  • Defined foo@@v1 (default version) resolves both undefined foo and foo@v1

If there are multiple default version definitions (such as foo@@v1 foo@@v2), a duplicate definition error should be issued even if one is weak. Usually a symbol has zero or one default version (@@) definition, and an arbitrary number of non-default version (@) definitions.

If the linker sees undefined foo and foo@v1 first, it will treat them as two symbols. When the linker see the definition foo@@v1, conceptually foo and foo@@v1 should be combined. If the linker sees foo@@v2 instead, foo@@v2 should resolve foo and foo@v1 should be a separate symbol.

  • Combining Versions describes the problem.
  • gold/symtab.cc Symbol_table::define_default_version uses a heuristic rule to solve this problem. It special cases on visibility, but I feel that this rule is unneeded.
  • Before 2.26, GNU ld reported a bogus multiple definition error for defined weak foo@@v1 and defined global foo@v1 PR ld/26978
  • Before 2.26, GNU ld had a bug that the visibility of undefined foo@v1 does not affect the output visibility of foo@@v1: PR ld/26979
  • I fixed the object file side problem of LLD 12.0 in https://reviews.llvm.org/D92259 foo Archive files and lazy object files may still have incompatibility issues.

When LLD sees a defined foo@@v, it adds both foo and foo@v1 into the symbol table, thus foo@@v1 can resolve both undefined foo and foo@v1. After processing all input files, a pass iterates symbols and redirects foo@v1 to foo@@v1. Becase LLD treats them as separate symbols during input processing, a defined foo@v cannot suppress the extraction of an archive member defining foo@@v1, leading to a behavior incompatible with GNU ld. This probably does not matter, though.

GNU ld has another strange behavior: if both foo and foo@v1 are defined, foo will be removed. I strongly believe it is an issue in GNU ld but the maintainer rejected PR ld/27210.

Version script

To define a versioned symbol in a shared object or an executable, a version script must be specified. If all versioned symbols are undefined, then the version script can be omitted.

# Make all symbols other than foo and bar local.
{ global: foo; bar; local: *; };

# Assign version FBSD_1.0 to malloc and version FBSD_1.3 to mallocx,
# and make internal local.
FBSD_1.0 { malloc; local: internal; };
FBSD_1.3 { mallocx; };

A version script has three purposes:

  • Define versions.
  • Specify some patterns so that matched defined symbols (which do not have @ in the name) are tied to the specified version.
  • Scope reduction: for a defined unversioned symbol matched by a local: pattern, its binding will be changed to STB_LOCAL and will not be exported to the dynamic symbol table.

A version script can consist of one anonymous version tag ({...};) or a list of named version tags (v1 {...};). If you use an anonymous version tag with other version tags, GNU ld will error: anonymous version tag cannot be combined with other version tags. A local: part can be placed in any version tag. Which version tag is used does not matter.

If a defined symbol is matched by multiple version tags, the following precedence rules apply (binutils-gdb/bfd/linker.c:find_version_for_sym):

  • The first version tag with an exact pattern (i.e. there is no wildcard) wins.
  • Otherwise, the last version tag with a non-* wildcard pattern wins.
  • Otherwise, the first version tag with a * pattern wins.

The gotcha is that ** is a wildcard pattern which matches any symbol but its precedence is higher than *.

Most patterns are exact so gold and LLD iterate patterns instead of symbols to improve performance.

How a versioned symbol is produced

An undefined symbol can be assigned a version if:

  • its name does not contain @ (.symver is unused) and a shared object provides a default version definition.
  • its name contains @ and a shared object defines the symbol. GNU ld errors if there is no such a shared object. After https://reviews.llvm.org/D92260, LLD will report an error as well.

A defined symbol can be assigned a version if:

  • its name does not contain @ and it is matched by a pattern in a named version tag in a version script.
  • its name contains @
    • If -shared, the version should be defined by a version script, otherwise GNU ld errors version node not found for symbol. This exception looks strange to me so I have filed PR ld/26980.
    • If -no-pie or -pie, a version definition is unneeded in GNU ld. This behavior is strange.

ld.so behavior

/Linux Standard Base Core Specification, Generic Part/ describes the behavior of ld.so. Kan added symbol versioning support to FreeBSD rtld in 2005.

The DT_VERNEED and DT_VERNEEDNUM tags in the dynamic table delimiter the version requirement by a shared object/executable file: the requires versions and required shared object names (Vernaux::vna_name).

For each Vernaux entry (a Verneed's auxilliary entry) without the VER_FLG_WEAK bit, ld.so checks whether the referenced shared object has the DT_VERDEF table. If no, ld.so handles the case as a graceful degradation; if yes and the table does not define the version, ld.so reports an error. [verneed-check]

Usually a minor release does not bump soname. Suppose that libB.so depends on the libA 1.3 (soname is libA.so.1) and calls an function which does not exist in libA 1.2. If PLT lazy binding is used, libB.so may seem to work on a system with libA 1.2, until the PLT of the 1.3 symbol is called. If symbol versioning is not used and you want to solve this problem, you have to record the minor version number (libA.so.1.3) in the soname. However, bumping soname is all-or-nothing: all the dependent shared objects need to be relinked. If symbol versioning is used, you can continue to use the soname libA.so.1. ld.so will report an error if libA 1.2 is used, because the 1.3 version required by libB.so does not exist.

In the symbol resolution stage:

  • An undefined foo can be resolved to a definition of foo or foo@@v2 (only the definitions with index number 1 (VER_NDX_GLOBAL) and 2 are used in the reference match).
  • An undefined foo@v1 can be resolved to a definition of foo, foo@v1, or foo@@v1.

Note (undefined foo resolving to foo@v1) is allowed by ld.so but not allowed by the linker {reject-non-default}. This difference provides a mechanism to refuse linking against old symbols while keeping compatibility with unversioned old libraries. If a new version of a shared object needs to deprecate an unversioned bar, you can remove bar and define bar@compat instead. Libraries using bar are unaffected but new links against bar are disallowed.

Upgraded symbols in glibc

Note that GNU nm before binutils 2.35 does not display @ or @@.

nm -D /lib/x86_64-linux-gnu/libc.so.6 | \
  awk '$2!="U" {i=index($3,"@"); if(i){v=substr($3,i); $3=substr($3,1,i-1); m[$3]=m[$3]" "v}} \
  END {for(f in m)if(m[f]~/@.+@/)print f, m[f]}'

The output on my x86-64 system:

pthread_cond_broadcast  @GLIBC_2.2.5 @@GLIBC_2.3.2
clock_nanosleep  @@GLIBC_2.17 @GLIBC_2.2.5
_sys_siglist  @@GLIBC_2.3.3 @GLIBC_2.2.5
sys_errlist  @@GLIBC_2.12 @GLIBC_2.2.5 @GLIBC_2.3 @GLIBC_2.4
quick_exit  @GLIBC_2.10 @@GLIBC_2.24
memcpy  @@GLIBC_2.14 @GLIBC_2.2.5
regexec  @GLIBC_2.2.5 @@GLIBC_2.3.4
pthread_cond_destroy  @GLIBC_2.2.5 @@GLIBC_2.3.2
nftw  @GLIBC_2.2.5 @@GLIBC_2.3.3
pthread_cond_timedwait  @@GLIBC_2.3.2 @GLIBC_2.2.5
clock_getres  @GLIBC_2.2.5 @@GLIBC_2.17
pthread_cond_signal  @@GLIBC_2.3.2 @GLIBC_2.2.5
fmemopen  @GLIBC_2.2.5 @@GLIBC_2.22
pthread_cond_init  @GLIBC_2.2.5 @@GLIBC_2.3.2
clock_gettime  @GLIBC_2.2.5 @@GLIBC_2.17
sched_setaffinity  @GLIBC_2.3.3 @@GLIBC_2.3.4
glob  @@GLIBC_2.27 @GLIBC_2.2.5
sys_nerr  @GLIBC_2.2.5 @GLIBC_2.4 @@GLIBC_2.12 @GLIBC_2.3
_sys_errlist  @GLIBC_2.3 @GLIBC_2.4 @@GLIBC_2.12 @GLIBC_2.2.5
sys_siglist  @GLIBC_2.2.5 @@GLIBC_2.3.3
clock_getcpuclockid  @GLIBC_2.2.5 @@GLIBC_2.17
realpath  @GLIBC_2.2.5 @@GLIBC_2.3
sys_sigabbrev  @GLIBC_2.2.5 @@GLIBC_2.3.3
posix_spawnp  @@GLIBC_2.15 @GLIBC_2.2.5
posix_spawn  @@GLIBC_2.15 @GLIBC_2.2.5
_sys_nerr  @@GLIBC_2.12 @GLIBC_2.4 @GLIBC_2.3 @GLIBC_2.2.5
nftw64  @GLIBC_2.2.5 @@GLIBC_2.3.3
pthread_cond_wait  @GLIBC_2.2.5 @@GLIBC_2.3.2
sched_getaffinity  @GLIBC_2.3.3 @@GLIBC_2.3.4
clock_settime  @GLIBC_2.2.5 @@GLIBC_2.17
glob64  @@GLIBC_2.27 @GLIBC_2.2.5
  • realpath@@GLIBC_2.3: the previous version returns EINVAL when the second parameter is NULL
  • memcpy@@GLIBC_2.14 BZ12518: the previous version guarantees a forward copying behavior. Shockwave Flash at that time had a "memcpy downward" bug which required the workaround.
  • quick_exit@@GLIBC_2.24 BZ20198: the previous version copies the destructors of thread_local objects.
  • glob64@@GLIBC_2.27: the previous version does not follow dangling symlinks.

How to remove symbol versioning

Imagine that you want to build an application with a prebuilt shared object which has versioned references, but you can only find shared objects providing the unversioned definitions. The linker will helpfully error:

ld.lld: error: undefined reference to foo@v1 [--no-allow-shlib-undefined]

As the diagnostic suggests, you can add --allow-shlib-undefined to get rid of the error. It is not recommended but the built application may happen to work.

For this case, an alternative hacky solution is:

# 32-bit
cp in.so out.so
r2 -wqc '/x feffff6f00000000 @ section..dynamic; w0 16 @ hit0_0' out.so
llvm-objcopy -R .gnu.version out.so

# 64-bit
cp in.so out.so
r2 -wqc '/x feffff6f @ section..dynamic; w0 8 @ hit0_0' out.so
llvm-objcopy -R .gnu.version out.so

With the removal of .gnu.version, the linker will think that out.so references foo instead of foo@v1. However, llvm-objcopy will zero out the section contents. At runtime, glibc ld.so will complain unsupported version 0 of Verneed record. To make glibc happy, you can delete DT_VER* tags from the dynamic table. The above code snippet uses an r2 command to locate DT_VERNEED(0x6ffffffe) and rewrite it to DT_NULL(a DT_NULL entry stops the parsing of the dynamic table). The difference of the readelf -d output is roughly:

  0x000000006ffffffb (FLAGS_1)            Flags: NOW
- 0x000000006ffffffe (VERNEED)            0x8ef0
- 0x000000006fffffff (VERNEEDNUM)         5
- 0x000000006ffffff0 (VERSYM)             0x89c0
- 0x000000006ffffff9 (RELACOUNT)          1536
  0x0000000000000000 (NULL)               0x0

LLD

  • If an undefined symbol is not defined by a shared object, GNU ld will report an error. LLD before 12.0 did not error (I fixed it in https://reviews.llvm.org/D92260).

Remarks

GCC/Clang supports asm specifier and #pragma redefine_extname renaming a symbol. For example, if you declare int foo() asm("foo_v1"); and then reference foo, the symbol in .o will be foo_v1.

For example, the biggest change in musl v1.2.0 is the time64 support for its supported 32-bit architectures. musl adopted a scheme based on asm specifiers:

// include/features.h
#define __REDIR(x,y) __typeof__(x) x __asm__(#y)

// API header include/sys/time.h
int utimes(cosnt char *, const struct timeval [2]);
__REDIR(utimes, __utimes_time64);

// Implementation src/linux/utimes.c
int utimes(const char *path, const struct timeval times[2]) { ... }

// Internal header compat/time32/time32.h
int __utimes_time32() __asm__("utimes");

// Compat implementation compat/time32/utimes_time32.c
int __utimes_time32(const char *path, const struct timeval32 times32[2]) { ... }
  • In .o, the time32 symbol remains utimes and is compatible with the ABI required by programs linked against old musl versions; the time64 symbol is __utimes_time64.
  • The public header redirects utimes to __utimes_time64.
    • cons: if the user declares utimes by themself, they will not link against the correct __utimes_time64.
  • The "good-looking" name utimes is used for the preferred time64 implementation internally and the "ugly" name __utimes_time32 is used for the legacy time32 implementation.
    • If the time32 implementation is called elsewhere, the "ugly" name can make it stand out.

For the above example, here is an implementation with symbol versioning:

// API header include/sys/time.h
int utimes(cosnt char *, const struct timeval [2]);

// Implementation src/linux/utimes.c
int utimes(const char *path, const struct timeval times[2]) { ... }

// Internal header compat/time32/time32.h
// Probably __asm__(".symver __utimes_time32, utimes@time32, rename"); if supported
__asm__(".symver __utimes_time32, utimes@time32");

// Implementation compat/time32/utimes_time32.c
int __utimes_time32(const char *path, const struct timeval32 times32[2])
{
  ...
}

Note that it is @@@ cannot be used. The header is included in a defining translation unit and @@@ will lead to a default version definition while we want a non-default version definition.

According to Assembler behavior, the undesirable __utimes_time32 is present. Be careful to use a version script to localize it.

So what is the significance of symbol versioning? I think carefully:

  • Refuse linking against old symbols while keeping compatibility with unversioned old libraries. {reject-non-default}
  • No need to label declarations.
  • The version definition can be delayed until link time. The version script provides a flexible pattern matching mechanism to assign versions.
  • Scope reduction. Arguably another mechanism like --dynamic-list might have been developed if version scripts did not provide local:.
  • There are some semantic issues in renaming builtin functions with asm specifiers in GCC and Clang (they do not know that the renamed symbol has built-in semantic). See 2020-10-15-intra-call-and-libc-symbol-renaming
  • [verneed-check]

For the first item, the asm specifier scheme uses conventions to prevent problems (users should include the header); and symbol versioning can be forced by ld.

Design flaws:

  • .symver foo, foo@v1 In foobehavior defined {gas-copy}: reserved symbol foo(redundant symbol has a link), binding / st_othersync (not convenient to set different binding / visibility)
  • Verdaux is a bit redundant. In practice, one Verdef has only one auxilliary Verdaux entry.
  • This is arguably a minor problem but annoying for a framework providing multiple shared objects. ld.so requires "a versioned symbol is implemented in the same shared object in which it was found at link time", which disallows moving definitions between shared objects. Fortunately, glibc 2.30 BZ24741 relaxes this requirement, essentially ignoring Vernaux::vna_name.

Before that, glibc used a forwarder to move clock_* functions from librt.so to libc.so:

// rt/clock-compat.c
__typeof(clock_getres) *clock_getres_ifunc(void) asm("clock_getres");
__typeof(clock_getres) *clock_getres_ifunc(void) { return &__clock_getres; }

libc.so defines __clock_getres and clock_getres. librt.so defines an ifunc called clock_getres which forwards to libc.so __clock_getres.

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