Align segment lengths after merging (#831)

* Align segment length after merging

* Explain the need for pad_alignment

CHANGELOG.md

@@ -47,6 +47,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Fixed typos in error variant names (#782)
 - Fix `read-flash` which didn't work with some lengths (#804)
 - espflash can now flash an ESP32-S2 in download mode over USB (#813)
+- Fixed a case where esplash transformed the firmware elf in a way that made it unbootable (#831)
 
 ### Removed
 

espflash/src/image_format/esp_idf.rs

@@ -213,8 +213,14 @@ impl<'a> IdfBootloaderFormat<'a> {
 
         let mut data = bytes_of(&header).to_vec();
 
-        let flash_segments: Vec<_> = merge_adjacent_segments(rom_segments(chip, &elf).collect());
-        let mut ram_segments: Vec<_> = merge_adjacent_segments(ram_segments(chip, &elf).collect());
+        // The bootloader needs segments to be 4-byte aligned, but ensuring that
+        // alignment by padding segments might result in overlapping segments. We
+        // need to merge adjacent segments first to avoid the possibility of them
+        // overlapping, and then do the padding.
+        let flash_segments: Vec<_> =
+            pad_align_segments(merge_adjacent_segments(rom_segments(chip, &elf).collect()));
+        let mut ram_segments: Vec<_> =
+            pad_align_segments(merge_adjacent_segments(ram_segments(chip, &elf).collect()));
 
         let mut checksum = ESP_CHECKSUM_MAGIC;
         let mut segment_count = 0;
@@ -446,6 +452,11 @@ fn merge_adjacent_segments(mut segments: Vec<Segment<'_>>) -> Vec<Segment<'_>> {
     merged
 }
 
+fn pad_align_segments(mut segments: Vec<Segment<'_>>) -> Vec<Segment<'_>> {
+    segments.iter_mut().for_each(|segment| segment.pad_align(4));
+    segments
+}
+
 /// Save a segment to the data buffer.
 fn save_flash_segment(
     data: &mut Vec<u8>,

espflash/src/image_format/mod.rs

@@ -33,13 +33,12 @@ pub struct Segment<'a> {
 
 impl<'a> Segment<'a> {
     pub fn new(addr: u32, data: &'a [u8]) -> Self {
-        let mut segment = Segment {
+        // Do not pad the data here, as it might result in overlapping segments
+        // in the ELF file. The padding should be done after merging adjacent segments.
+        Segment {
             addr,
             data: Cow::Borrowed(data),
-        };
-        segment.pad_align(4);
-
-        segment
+        }
     }
 
     /// Split of the first `count` bytes into a new segment, adjusting the