v0.3: ELF symbol parsing, sym()/addr()/symbols() globals, addrstr/here prelude helpers

Cargo.lock

@@ -77,9 +77,16 @@ version = "0.1.0"
 dependencies = [
  "libc",
  "nix",
+ "object",
  "rquickjs",
 ]
 
+[[package]]
+name = "memchr"
+version = "2.8.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6b947ae49db0d222b1dbc6b113ce7248a3fc3a6ca21b696717bfc000ba4484d8"
+
 [[package]]
 name = "nix"
 version = "0.31.3"
@@ -92,6 +99,15 @@ dependencies = [
  "libc",
 ]
 
+[[package]]
+name = "object"
+version = "0.36.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "62948e14d923ea95ea2c7c86c71013138b66525b86bdc08d2dcc262bdb497b87"
+dependencies = [
+ "memchr",
+]
+
 [[package]]
 name = "proc-macro2"
 version = "1.0.106"

Cargo.toml

@@ -7,3 +7,4 @@ edition = "2024"
 nix = { version = "0.31", features = ["ptrace", "process", "uio", "signal"] }
 rquickjs = { version = "0.12", features = ["std"] }
 libc = "0.2"
+object = { version = "0.36", default-features = false, features = ["read", "std"] }

src/js.rs

@@ -318,6 +318,53 @@ fn install_globals<'js>(
         globals.set("prev", prev_obj)?;
     }
 
+    // ---------- sym(addr) -> "name+0xN" | null ----------
+    {
+        let target = target.clone();
+        let f = Function::new(c.clone(), move |c: Ctx<'js>, addr: Value<'js>| -> rquickjs::Result<Value<'js>> {
+            let addr = coerce_addr(&c, &addr)?;
+            let slot = target.borrow();
+            let t = slot.as_ref().ok_or_else(|| throw(&c, "no target attached"))?;
+            let s = t.symbols().ok_or_else(|| throw(&c, "no symbol table"))?;
+            match s.name_for_addr(addr) {
+                Some(name) => Ok(rquickjs::String::from_str(c.clone(), &name)?.into_value()),
+                None => Ok(Value::new_null(c.clone())),
+            }
+        })?;
+        globals.set("sym", f)?;
+    }
+
+    // ---------- addr(name) -> BigInt | null ----------
+    {
+        let target = target.clone();
+        let f = Function::new(c.clone(), move |c: Ctx<'js>, name: String| -> rquickjs::Result<Value<'js>> {
+            let slot = target.borrow();
+            let t = slot.as_ref().ok_or_else(|| throw(&c, "no target attached"))?;
+            let s = t.symbols().ok_or_else(|| throw(&c, "no symbol table"))?;
+            match s.addr_for_name(&name) {
+                Some(a) => u64_to_bigint(&c, a),
+                None => Ok(Value::new_null(c.clone())),
+            }
+        })?;
+        globals.set("addr", f)?;
+    }
+
+    // ---------- symbols() -> { count, load_base, path } ----------
+    {
+        let target = target.clone();
+        let f = Function::new(c.clone(), move |c: Ctx<'js>| -> rquickjs::Result<Value<'js>> {
+            let slot = target.borrow();
+            let t = slot.as_ref().ok_or_else(|| throw(&c, "no target attached"))?;
+            let s = t.symbols().ok_or_else(|| throw(&c, "no symbol table"))?;
+            let obj = Object::new(c.clone())?;
+            obj.set("count", s.len() as u32)?;
+            obj.set("load_base", u64_to_bigint(&c, s.load_base)?)?;
+            obj.set("path", s.path.to_string_lossy().into_owned())?;
+            Ok(obj.into_value())
+        })?;
+        globals.set("symbols", f)?;
+    }
+
     // ---------- JS-side prelude ---------------------------------------------
     // Loaded last, so it can reference any of the native globals above.
     // Lives in src/prelude.js, embedded at compile time.

src/main.rs

@@ -10,6 +10,7 @@
 //!   - EOF on stdin exits cleanly.
 
 mod target;
+mod symbols;
 mod js;
 
 use std::io::{BufRead, Write};

src/prelude.js

@@ -85,3 +85,29 @@ globalThis.repr = function (v) {
     }
     return String(v);
 };
+
+// ---------------------------------------------------------------------------
+// addrstr(v) — render an address with symbol context.
+// Returns "0x7fff... (main+0x4)" if the address is inside a known symbol,
+// or just "0x7fff..." otherwise. Safe to call before a target is attached:
+// falls back to bare hex if `sym` throws.
+
+globalThis.addrstr = function (v) {
+    const h = hex(v);
+    try {
+        const s = sym(v);
+        return s ? `${h} (${s})` : h;
+    } catch (_) {
+        return h;
+    }
+};
+
+// ---------------------------------------------------------------------------
+// here() — quick "where am I" one-liner. Prints pc with symbol context.
+
+globalThis.here = function (label) {
+    const s = addrstr(state.pc);
+    if (label) print(label, s); else print(s);
+};
+
+

src/symbols.rs

@@ -0,0 +1,195 @@
+//! ELF symbol table parsing and address lookups for the main executable.
+//!
+//! v0.3 scope:
+//! - Parse the main binary's symbol table (.symtab + .dynsym).
+//! - Handle PIE (ET_DYN) by reading the load base from /proc/<pid>/maps.
+//! - Provide name → addr and addr → "name+0xoff" lookups.
+//! - Shared libraries are NOT covered — those need ld.so awareness and live
+//!   in a later milestone.
+
+use object::{Object, ObjectSymbol, ObjectSection, SymbolKind};
+use std::collections::BTreeMap;
+use std::fs;
+use std::path::PathBuf;
+
+/// One named symbol with a load-base-adjusted virtual address and a size.
+/// Size==0 is common (assembly globals etc.); we treat 0 as "unknown size,
+/// match only by exact start address" when doing addr → name lookups.
+#[derive(Debug, Clone)]
+pub struct Symbol {
+    pub name: String,
+    pub addr: u64,
+    pub size: u64,
+}
+
+pub struct SymbolTable {
+    /// addr → symbol. Keys are post-relocation virtual addresses.
+    by_addr: BTreeMap<u64, Symbol>,
+    /// Lowercase name → addr. We keep the case-sensitive name in Symbol but
+    /// allow case-insensitive lookups since CLI debugger users won't always
+    /// remember exact capitalisation. Stores the *first* address seen for
+    /// a name (binaries can have duplicates after stripping).
+    by_name: BTreeMap<String, u64>,
+    /// Load base offset that was applied to file-relative addresses. 0 for
+    /// non-PIE binaries.
+    pub load_base: u64,
+    /// Path the symbols came from, for diagnostics.
+    pub path: PathBuf,
+}
+
+impl SymbolTable {
+    /// Load symbols from /proc/<pid>/exe, applying the load base from
+    /// /proc/<pid>/maps if the binary is PIE.
+    pub fn from_pid(pid: i32) -> Result<SymbolTable, String> {
+        let exe_path = read_exe_path(pid)?;
+        let load_base = read_load_base(pid, &exe_path)?;
+        let bytes = fs::read(&exe_path)
+            .map_err(|e| format!("read exe {}: {}", exe_path.display(), e))?;
+        let elf = object::File::parse(&*bytes)
+            .map_err(|e| format!("parse ELF: {}", e))?;
+
+        let mut by_addr: BTreeMap<u64, Symbol> = BTreeMap::new();
+        let mut by_name: BTreeMap<String, u64> = BTreeMap::new();
+
+        for sym in elf.symbols().chain(elf.dynamic_symbols()) {
+            if !matches!(sym.kind(), SymbolKind::Text | SymbolKind::Data | SymbolKind::Unknown) {
+                continue;
+            }
+            let raw_name = match sym.name() {
+                Ok(n) if !n.is_empty() => n,
+                _ => continue,
+            };
+            // Filter out section-undefined symbols (imports).
+            if sym.section_index().is_none() && !sym.is_definition() {
+                continue;
+            }
+            let addr = sym.address().wrapping_add(load_base);
+            if addr == 0 { continue; }
+            let s = Symbol {
+                name: raw_name.to_string(),
+                addr,
+                size: sym.size(),
+            };
+            // Prefer the entry already there if both exist at same address
+            // and the newer one has size 0 — keeps size info around.
+            match by_addr.get(&addr) {
+                Some(existing) if existing.size > 0 && s.size == 0 => { /* keep existing */ }
+                _ => { by_addr.insert(addr, s.clone()); }
+            }
+            by_name.entry(raw_name.to_string()).or_insert(addr);
+            // Also stash a lowercase alias if it isn't already a key
+            let lc = raw_name.to_lowercase();
+            if lc != raw_name {
+                by_name.entry(lc).or_insert(addr);
+            }
+        }
+
+        // For diagnostic: also note executable section ranges (might help us
+        // later filter "is this addr even code"). Not used yet but cheap.
+        let _exec_sections: Vec<_> = elf.sections()
+            .filter(|s| s.kind() == object::SectionKind::Text)
+            .map(|s| (s.address().wrapping_add(load_base), s.size()))
+            .collect();
+
+        Ok(SymbolTable { by_addr, by_name, load_base, path: exe_path })
+    }
+
+    /// Find the symbol containing or starting at `addr`.
+    /// Returns "name" if exact, "name+0xN" if interior, None if no match.
+    pub fn name_for_addr(&self, addr: u64) -> Option<String> {
+        // Look at the symbol whose addr ≤ ours, and check whether ours falls
+        // within [sym.addr, sym.addr + sym.size). For size==0 symbols, only
+        // match an exact equality.
+        let (&_, sym) = self.by_addr.range(..=addr).next_back()?;
+        let off = addr - sym.addr;
+        if sym.size == 0 {
+            if off == 0 { Some(sym.name.clone()) } else { None }
+        } else if off < sym.size {
+            if off == 0 { Some(sym.name.clone()) } else { Some(format!("{}+0x{:x}", sym.name, off)) }
+        } else {
+            None
+        }
+    }
+
+    /// Resolve a name to a single address. Tries exact case first, then
+    /// lowercase. Returns None if not found or ambiguous.
+    pub fn addr_for_name(&self, name: &str) -> Option<u64> {
+        if let Some(&a) = self.by_name.get(name) { return Some(a); }
+        self.by_name.get(&name.to_lowercase()).copied()
+    }
+
+    pub fn len(&self) -> usize {
+        self.by_addr.len()
+    }
+}
+
+fn read_exe_path(pid: i32) -> Result<PathBuf, String> {
+    fs::read_link(format!("/proc/{}/exe", pid))
+        .map_err(|e| format!("read /proc/{}/exe: {}", pid, e))
+}
+
+/// For a PIE (ET_DYN) binary, find the first executable mapping of the exe
+/// file in /proc/<pid>/maps and use its address as the load base. For non-PIE
+/// (ET_EXEC), the load base is 0.
+fn read_load_base(pid: i32, exe_path: &PathBuf) -> Result<u64, String> {
+    // Peek at the ELF header e_type via the file we'll parse anyway.
+    let bytes = fs::read(exe_path)
+        .map_err(|e| format!("read exe {}: {}", exe_path.display(), e))?;
+    let elf = object::File::parse(&*bytes)
+        .map_err(|e| format!("parse ELF: {}", e))?;
+    // object doesn't expose e_type directly on the high-level File, but
+    // it does expose `relative_address_base` which is what we want: for
+    // ET_EXEC this is the absolute base of the file's address space,
+    // for ET_DYN it's 0 (so symbol addresses are file-offsets-as-vaddrs).
+    // We compute load base = first executable mapping start - (file's PT_LOAD
+    // p_vaddr for that segment). Easier: parse /proc/maps and look for the
+    // first executable mapping whose pathname matches exe_path.
+    let exe_str = exe_path.to_string_lossy().into_owned();
+    let maps = fs::read_to_string(format!("/proc/{}/maps", pid))
+        .map_err(|e| format!("read /proc/{}/maps: {}", pid, e))?;
+
+    // /proc/<pid>/maps line format:
+    //   ADDR_LO-ADDR_HI PERMS OFFSET DEV INODE      PATH
+    // We want the first line where PATH matches our exe AND PERMS contains 'x'.
+    for line in maps.lines() {
+        // Split into at most 6 whitespace-delimited fields; the last is the
+        // path which may itself contain spaces (unlikely in practice).
+        let mut it = line.splitn(6, ' ').filter(|s| !s.is_empty());
+        let range = match it.next() { Some(s) => s, None => continue };
+        let perms = match it.next() { Some(s) => s, None => continue };
+        let _offset = it.next();
+        let _dev = it.next();
+        let _inode = it.next();
+        let path = it.next().unwrap_or("").trim();
+        if !perms.contains('x') { continue; }
+        if path != exe_str { continue; }
+        let mut parts = range.split('-');
+        let lo = parts.next().ok_or_else(|| "bad maps line".to_string())?;
+        let lo = u64::from_str_radix(lo, 16).map_err(|e| format!("bad maps addr: {}", e))?;
+
+        // For ET_DYN this is the load base directly (file offset 0 maps to lo).
+        // For ET_EXEC, file vaddrs are absolute, and lo equals the first
+        // PT_LOAD's p_vaddr, so we want 0.
+        if is_pie(&elf) {
+            // We assume the first executable mapping starts at file offset 0
+            // for the .text segment — usually true. If we wanted to be careful
+            // we'd subtract the segment's file offset, but it's normally 0
+            // for the first executable PT_LOAD.
+            return Ok(lo);
+        } else {
+            return Ok(0);
+        }
+    }
+    // Fallback: no executable mapping found. Probably never happens.
+    Ok(0)
+}
+
+fn is_pie(elf: &object::File) -> bool {
+    // ET_DYN is what gcc produces by default for executables ("-pie") and
+    // also for shared libraries. For an executable file, ET_DYN ≡ PIE.
+    match elf {
+        object::File::Elf64(e) => e.elf_header().e_type.get(object::Endianness::Little) == object::elf::ET_DYN,
+        object::File::Elf32(e) => e.elf_header().e_type.get(object::Endianness::Little) == object::elf::ET_DYN,
+        _ => false,
+    }
+}

src/target.rs

@@ -11,6 +11,8 @@ use nix::unistd::{execvp, fork, ForkResult, Pid};
 use std::ffi::CString;
 use std::io::IoSliceMut;
 
+use crate::symbols::SymbolTable;
+
 /// Why the target stopped after a step/continue, or that it has exited.
 #[derive(Debug, Clone)]
 pub enum StopReason {
@@ -55,6 +57,9 @@ pub struct Target {
     /// Snapshot of registers from the previous stop. None until at least one
     /// step/cont has happened after spawn.
     prev_regs: Option<libc::user_regs_struct>,
+    /// ELF symbols of the main executable. None if parsing failed (e.g.
+    /// stripped binary, unreadable /proc) — symbol lookups become no-ops.
+    symbols: Option<SymbolTable>,
 }
 
 impl Target {
@@ -102,11 +107,15 @@ impl Target {
                 let status = waitpid(child, None)
                     .map_err(|e| format!("waitpid (initial) failed: {}", e))?;
                 match status {
-                    WaitStatus::Stopped(_, Signal::SIGTRAP) => Ok(Target {
+                    WaitStatus::Stopped(_, Signal::SIGTRAP) => {
+                        let symbols = SymbolTable::from_pid(child.as_raw()).ok();
+                        Ok(Target {
                             pid: child,
                             state: State::Stopped(StopReason::Step),
                             prev_regs: None,
-                    }),
+                            symbols,
+                        })
+                    }
                     WaitStatus::Exited(_, code) => {
                         Err(format!("child exited immediately with code {} (exec failed?)", code))
                     }
@@ -143,6 +152,10 @@ impl Target {
         self.prev_regs
     }
 
+    pub fn symbols(&self) -> Option<&SymbolTable> {
+        self.symbols.as_ref()
+    }
+
     fn wait_for_stop(&mut self) -> Result<StopReason, String> {
         let status = waitpid(self.pid, None).map_err(|e| format!("waitpid: {}", e))?;
         let reason = match status {