[2/10] libs/libc/elf: Give a shared object a word alignment, not zero.

[casaroli]

Summary

libelf_elfsize() takes textalign and dataalign from the section headers, which only the ET_REL path walks. An ET_DYN object is sized from its program headers instead, so both stay at zero, and the allocation a few lines later asks lib_memalign() for that alignment.

Zero is not a valid alignment, and every path that receives it divides by it. mm_memalign() accepts zero as a power of two, because 0 & -0 is 0, then takes the alignment <= MM_ALIGN branch and evaluates ((uintptr_t)ptr) % alignment in a DEBUGASSERT. With CONFIG_MM_HEAP_MEMPOOL and a pool that fits the request the object never reaches that branch and gets ALIGN_UP(blk, 0) instead, which is ((blk - 1) / 0) * 0.

On Cortex-M this is usually invisible: UDIV returns zero for a division by zero unless CCR.DIV_0_TRP is set, which NuttX does not set. It is a SIGFPE on the simulator, and the mempool path returns a null pointer wherever the division yields zero, which the loader reports as -ENOMEM.

Ask for a natural word when the program headers gave nothing. p_align is the linker's page granularity, not a section requirement, so honouring it would cost a page per module for no gain.

This is the second of ten PRs that #19673 is being split into, so each can be reviewed on its own. The first four have no FDPIC content at all and do not depend on each other, so they can merge in any order. The remaining six are the FDPIC work itself, one subsystem each, and each is a no-op with CONFIG_FDPIC off: loader core, ARM relocations, the callback entry points, the exec() path, DT_NEEDED through dlopen(), then documentation and a board configuration. The others are [1/10] #19938, [3/10] #19940 and [4/10] #19941.

Impact

Every ET_DYN module, which is every shared object opened with dlopen(), is allocated through an aligned path rather than a zero one.

No configuration change, no size change beyond the alignment of one allocation.

Testing

Built for mps3-an547:picostest, which is CONFIG_ELF with CONFIG_PIC. tools/checkpatch.sh passes.

Runtime evidence on real hardware follows tomorrow, on an RP2350.

[github-actions]

MemBrowse Memory Report

s698pm-dkit

• Code: .text +64 B (+0.0%, 363,888 B)
  No memory changes detected for:
• arduino-mega2560
• esp32-devkitc
• hifive1-revb
• mirtoo
• qemu-armv8a
• qemu-intel64
• rx65n-rsk2mb
• stm32-nucleo-f103rb