From a5f280c4c67f53b4863d850bd3942be649bc9205 Mon Sep 17 00:00:00 2001
From: Joakim Hulthe <joakim@hulthe.net>
Date: Wed, 10 Jul 2024 23:33:19 +0200
Subject: [PATCH] Take SHT_NOBITS into account

---
 src/main.rs    |  8 +++++++-
 src/structs.rs | 24 ++++++++++++++++++++++++
 2 files changed, 31 insertions(+), 1 deletion(-)

diff --git a/src/main.rs b/src/main.rs
index 45cffb8..46330f4 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -95,8 +95,11 @@ macro_rules! parse_elf {
             let name = &names_section[name_start..name_end];
             let name = from_utf8(name).wrap_err(eyre!(".names entry not valid utf-8"))?;
             let section_start = entry.start().wrap_err_with(|| name.to_string())?;
+
+            // validate that section fits in file (or has SHT_NOBITS set)
+            let sht_nobits_set = (entry.sh_type.to_native() & 8) != 0;
             let section_end = entry.end().wrap_err_with(|| name.to_string())?;
-            if section_end > elf.len() {
+            if section_end > elf.len() && !sht_nobits_set {
                 bail!("section {name:?} end is past eof");
             }
 
@@ -190,6 +193,7 @@ fn main() -> eyre::Result<()> {
 
     let elf = fs::read(opt.elf).wrap_err("failed to read elf file")?;
 
+    // extract first part of the header
     if elf.len() < HEADER1_LEN {
         bail!("not a valid elf file: file too small");
     }
@@ -199,6 +203,7 @@ fn main() -> eyre::Result<()> {
     let mut out = Output::default();
     out.write("header", &header)?;
 
+    // validate header
     if &header.ei_magic != b"\x7FELF" {
         bail!("not a valid elf file: invalid magic");
     }
@@ -207,6 +212,7 @@ fn main() -> eyre::Result<()> {
         bail!("unknown elf version: 0x{:x}", header.ei_version);
     }
 
+    // determine endianness and size, then invoke the correct parse macro.
     match (header.ei_class, header.ei_data) {
         (1, 1) => parse_elf!(LittleEndian32, &mut out, header1, elf),
         (2, 1) => parse_elf!(LittleEndian64, &mut out, header1, elf),
diff --git a/src/structs.rs b/src/structs.rs
index af6bd5f..b6d760b 100644
--- a/src/structs.rs
+++ b/src/structs.rs
@@ -50,6 +50,7 @@ impl ElfConfig for BigEndian64 {
     type SymtabEntry = SymbolTableEntry64<BigEndian>;
 }
 
+/// First part of an ELF header. Use to determine endian and word size of elf file.
 #[derive(Pod, Zeroable, Clone, Copy, Debug, Serialize)]
 #[repr(C, packed)]
 pub struct ElfHeader1 {
@@ -63,6 +64,7 @@ pub struct ElfHeader1 {
     _padding: [u8; 7],
 }
 
+/// Second part of an elf header. Size and endianness depends on [ElfHeader1].
 #[derive(Pod, Zeroable, Clone, Copy, Debug, Serialize)]
 #[repr(C, packed)]
 pub struct ElfHeader2<C: ElfConfig> {
@@ -84,15 +86,37 @@ pub struct ElfHeader2<C: ElfConfig> {
 #[derive(Pod, Zeroable, Clone, Copy, Debug, Serialize)]
 #[repr(C, packed)]
 pub struct SectionHeaderEntry<C: ElfConfig> {
+    /// An index into the section header string table containing the name of this section.
     pub sh_name: C::Word,
+
+    /// Specifies the section's semantics.
     pub sh_type: C::Word,
+
+    /// Bitflags that describe the s.ections attributes.
     pub sh_flags: C::Addr,
+
+    /// If the section will appear in the memory image of a process, this gives the address at
+    /// which the section should start. Otherwise, this is 0.
     pub sh_addr: C::Addr,
+
+    /// This member's value gives the byte offset from the beginning of the ELF file to the start
+    /// of this section. If `SHT_NOBITS` is set in sh_flags, the section occupies no space in the
+    /// file.
     pub sh_offset: C::Addr,
+
+    /// The size of the section in bytes.
     pub sh_size: C::Addr,
+
+    /// Section header table link. Interpretation depends on section type.
     pub sh_link: C::Word,
+
+    /// Extra section info. Interpretation depends on section type.
     pub sh_info: C::Word,
+
+    /// Address alignment constraints.
     pub sh_addralign: C::Addr,
+
+    /// Size in bytes of each entry in the section, or 0 if section does not contain entries.
     pub sh_entsize: C::Addr,
 }