Skip to content

Case 127: Exported Data Object Size Change

Field Value
Verdict 🔴 BREAKING
Category Breaking
Platforms Linux
Flags ABI break, Bad practice
Detected ChangeKinds symbol_size_changed
Source files examples/case127_data_object_size_changed/

Category: Symbol / Data Layout | Verdict: 🔴 BREAKING

What breaks

The library exports a global data object, config_table. In v1 it is int[16] (64 bytes); in v2 it grows to int[32] (128 bytes). The exported symbol's st_size changes.

This is the exact class of regression the glibc testsuite guards with its static-variable-size checks, and that Apple's dynamic-library guidelines classify as a major change ("changing a symbol's size"). It is detected at L0 — the size lives in the ELF symbol table, so no debug info is required.

Why it is an ABI break

When an executable references an exported data object, the static linker emits a copy relocation: it reserves space for the object in the executable's own BSS, sized using the v1 definition (64 bytes), and the dynamic loader copies the library's initial value into that fixed-size slot at startup.

After upgrading to v2:

  • The reserved copy in the old executable is still 64 bytes.
  • Any library code that indexes config_table[16..31] now reads or writes past the consumer's 64-byte copy — silent out-of-bounds memory access.
  • The two definitions disagree on the object's size, which is undefined behavior across the ABI boundary.

Unlike a removed function (which fails loudly at load), a grown data object corrupts memory silently, making this a particularly dangerous break.

Code diff

v1 v2
int config_table[16]; int config_table[32];
st_size = 64 st_size = 128

How to fix

  • Do not change the size of an exported data object within a SONAME. Bump the SONAME (major version) if the layout must change.
  • Better: never export mutable global arrays. Expose accessor functions (config_get / config_set) and keep the storage private (Apple's explicit recommendation), so its size is an implementation detail.

--policy-file glibc_symbol_versioned pins symbol-table regressions like this for versioned system libraries.

Source files

  • CMakeLists.txt
  • app.c
  • v1.c
  • v1.h
  • v2.c
  • v2.h

See also: Examples overview · All BREAKING cases · Category: Breaking.