libs/libc/elf: Give a shared object a word alignment, not zero.

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 fields stay at zero, and the allocation
a few lines later asks for that alignment:

  loadinfo->textalloc = lib_memalign(loadinfo->textalign, ...);

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, so the
assertion compares zero against zero and passes.  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, and the sections of a shared
object need no more than a word.

Built for mps3-an547:picostest, which is CONFIG_ELF with CONFIG_PIC.
Runtime evidence on hardware follows.

Assisted-by: Claude Opus 5 (1M context) <noreply@anthropic.com>
Signed-off-by: Marco Casaroli <marco.casaroli@gmail.com>

Author: casaroli

libs/libc/elf/elf_load.c

@@ -239,6 +239,20 @@ static void libelf_elfsize(FAR struct mod_loadinfo_s *loadinfo, bool alloc)
         }
     }
 
+  /* An ET_DYN object is sized from its program headers, which give no
+   * section alignment.  A word is enough.
+   */
+
+  if (loadinfo->textalign == 0)
+    {
+      loadinfo->textalign = sizeof(uintptr_t);
+    }
+
+  if (loadinfo->dataalign == 0)
+    {
+      loadinfo->dataalign = sizeof(uintptr_t);
+    }
+
   /* Save the allocation size */
 
   loadinfo->textsize = textsize;