[8/10] binfmt/elf: Load FDPIC modules through the ELF loader by casaroli · Pull Request #20131 · apache/nuttx

casaroli · 2026-09-13T18:48:54Z

Summary

[5/10] #19942, [6/10] #20089 and [7/10] #20130 are merged: an FDPIC object is placed, bound and relocated, and the firmware can call back into it. This makes exec() of one work.

The task needs the module's data base in its PIC base register. binfmt builds a D-Space for any object with a GOT, taking the base from the .got address; an FDPIC object names it in DT_PLTGOT instead, which the loader has already translated. The two are the same idea reached by different routes, and both are what up_initial_state() installs.

Constructors are not binfmt's business. A module carries its own crt0, which walks .init_array on the task that runs the module and then calls main, so they run in the module's own context with its own data base. For a module arriving through dlopen(), libelf_insert() walks the array instead, entering each entry through fdpic_invoke(), because a descriptor resolved on the calling task carries the wrong base.

The read-only segment of a module that executes in place is held by a filesystem pin. The load takes it, and the module owns it from the point where nothing can fail any more; it is given back when the task exits. The pin is held through a reference to the file rather than a descriptor, because the descriptor belongs to the task that called the loader while the release happens on another.

libelf_remove() and libelf_uninit() give back what an FDPIC module holds: the pin, and the writable segment. The read-only one is media rather than an allocation and must not be freed.

Impact

Behind CONFIG_FDPIC, which defaults off. With it off, binfmt is what it was.

Testing

mps3-an547:picostest builds on master with CONFIG_FDPIC off and on. With it on, the apps the configuration carries come out as FDPIC objects (OS/ABI: ARM FDPIC), which is what this patch loads.

The constructor and destructor loops enter the module through fdpic_invoke(), which [7/10] defines and which is now in master.

tools/checkpatch.sh -c -u -m -g passes.

Review

Everything below this in the series is merged, so it stands on master by itself.

github-actions · 2026-09-13T18:57:17Z

MemBrowse Memory Report

s698pm-dkit

Code: .text +256 B (+0.1%, 368,656 B)
No memory changes detected for:
arduino-mega2560
esp32-devkitc
hifive1-revb
mirtoo
qemu-armv8a
qemu-intel64
rx65n-rsk2mb
stm32-nucleo-f103rb

github-actions · 2026-09-13T22:55:21Z

🔗 Cross-repo PR dependencies

The read-only Build run reported the following dependent PR(s) and fetched head SHA(s):

[6/10] libs/libc/machine/arm: Relocate FDPIC function descriptors #20089 @ 74836bc
[7/10] libc, sched: Resolve FDPIC descriptors at module callback entry points #20130 @ 75cc4ef

CI run: https://github.com/apache/nuttx/actions/runs/34775786256

github-actions · 2026-09-15T15:46:38Z

🔗 Cross-repo PR dependencies

The read-only Build run reported the following dependent PR(s) and fetched head SHA(s):

[7/10] libc, sched: Resolve FDPIC descriptors at module callback entry points #20130 @ 3426186

CI run: https://github.com/apache/nuttx/actions/runs/34957863761

github-actions · 2026-09-16T06:37:11Z

🔗 Cross-repo PR dependencies

The read-only Build run reported the following dependent PR(s) and fetched head SHA(s):

[7/10] libc, sched: Resolve FDPIC descriptors at module callback entry points #20130 @ 98f9b2a

CI run: https://github.com/apache/nuttx/actions/runs/35060360729

xiaoxiang781216 · 2026-09-25T19:05:19Z

+
+          if (modp->xipbase == 0)
+            {
+#if defined(CONFIG_ARCH_USE_TEXT_HEAP)


#ifdef CONFIG_ARCH_USE_TEXT_HEAP

xiaoxiang781216 · 2026-09-25T19:06:50Z

+   * unloading thread does not carry.
+   */
+
+  gotbase = modp->fdpic ? modp->gotbase : 0;


why need? gotbase should be zero if fdpic equals false.

xiaoxiang781216 · 2026-09-25T19:07:18Z

  for (i = 0; i < modp->nfini; i++)
    {
-      array[i]();
+      fdpic_call(array[i], gotbase);


gotbase->modp->gotbase

xiaoxiang781216 · 2026-09-25T19:07:45Z

+   * none of its own.
+   */
+
+  gotbase = loadinfo.fdpic ? loadinfo.gotbase : 0;


xiaoxiang781216 · 2026-09-25T19:08:51Z

          case DT_PLTRELSZ:
            reldata.relsz[I_PLT] = dyn[i].d_un.d_val;
            break;
+          case DT_NEEDED:


let's move to DT_NEEDED patch

xiaoxiang781216 · 2026-09-25T19:09:27Z

+ *
+ ****************************************************************************/
+
+static inline void fdpic_call(CODE void (*fn)(void), uintptr_t got)


should we pass arg like fdpic_invoke

xiaoxiang781216 · 2026-09-25T19:10:02Z

  uint16_t nsect;                      /* Number of entries in sectalloc array */
 #endif
  int dynamic;                         /* Module is a dynamic shared object */
+  bool fdpic;                          /* Module is an FDPIC object: its two


exec() of an FDPIC module now works. The loader already places such an object and binds it; what was missing is everything binfmt has to carry across from the load to the running task. The task needs the module's data base in its PIC base register. binfmt builds a D-Space for any object with a GOT, taking the base from the .got section address; an FDPIC object names it in DT_PLTGOT instead, which the loader has already translated, so the two are the same idea reached by different routes and both are what up_initial_state() installs. Constructors are not binfmt's business. A module carries its own crt0, which walks .init_array on the task that runs the module and then calls main, so they run in the module's own context and with its own data base. For a module that arrives through dlopen(), libelf_insert() walks the array instead, and it enters each entry through fdpic_invoke() because a descriptor resolved on the calling task carries the wrong base. The read-only segment of a module that executes in place is held by a filesystem pin. The load takes it, and the module owns it from the point where nothing can fail any more; it is given back when the task that runs the module exits. The pin is held through a reference to the file rather than a descriptor, because the descriptor belongs to the task that called the loader and the release happens on another one. libelf_remove() and libelf_uninit() give back what an FDPIC module holds: the pin, and the writable segment, while the read-only one is media rather than an allocation and must not be freed. Built for mps3-an547:picostest with CONFIG_FDPIC both ways. Assisted-by: Claude Opus 5 (1M context) <noreply@anthropic.com> Signed-off-by: Marco Casaroli <marco.casaroli@gmail.com>

github-actions Bot added Size: M The size of the change in this PR is medium Area: BINFMT labels Sep 13, 2026

casaroli mentioned this pull request Sep 13, 2026

[10/10] Documentation, boards/rp23xx: Describe FDPIC and add a test configuration #20132

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acassis previously approved these changes Sep 14, 2026

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acassis marked this pull request as ready for review September 14, 2026 21:54

acassis requested review from anchao and xiaoxiang781216 as code owners September 14, 2026 21:54

casaroli dismissed acassis’s stale review via cefa2eb September 15, 2026 10:25

casaroli force-pushed the fdpic-exec branch from 59f570a to cefa2eb Compare September 15, 2026 10:25

casaroli marked this pull request as draft September 15, 2026 10:45

casaroli force-pushed the fdpic-exec branch from cefa2eb to 3a2ce9e Compare September 16, 2026 05:39

casaroli force-pushed the fdpic-exec branch from 3a2ce9e to 27a868a Compare September 25, 2026 13:51

casaroli marked this pull request as ready for review September 25, 2026 13:51

casaroli mentioned this pull request Sep 25, 2026

[9/10] libs/libc/elf: Load DT_NEEDED libraries with dlopen() #20368

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acassis previously approved these changes Sep 25, 2026

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casaroli dismissed acassis’s stale review via 03827a0 September 25, 2026 14:08

casaroli force-pushed the fdpic-exec branch from 27a868a to 03827a0 Compare September 25, 2026 14:08

casaroli requested a review from acassis September 25, 2026 14:17

acassis previously approved these changes Sep 25, 2026

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xiaoxiang781216 reviewed Sep 25, 2026

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casaroli dismissed acassis’s stale review via 39614ff September 26, 2026 06:59

casaroli force-pushed the fdpic-exec branch from 03827a0 to 39614ff Compare September 26, 2026 06:59

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[8/10] binfmt/elf: Load FDPIC modules through the ELF loader - #20131

[8/10] binfmt/elf: Load FDPIC modules through the ELF loader#20131
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