printf: handle extremely large format widths gracefully to fix GNU test panic

src/uucore/src/lib/features/format/mod.rs

@@ -113,7 +113,7 @@ impl Display for FormatError {
             Self::InvalidPrecision(precision) => write!(f, "invalid precision: '{precision}'"),
             // TODO: Error message below needs some work
             Self::WrongSpecType => write!(f, "wrong % directive type was given"),
-            Self::IoError(_) => write!(f, "io error"),
+            Self::IoError(_) => write!(f, "write error"),
             Self::NoMoreArguments => write!(f, "no more arguments"),
             Self::InvalidArgument(_) => write!(f, "invalid argument"),
             Self::MissingHex => write!(f, "missing hexadecimal number in escape"),
@@ -127,6 +127,25 @@ impl Display for FormatError {
     }
 }
 
+/// Maximum width for formatting to prevent memory allocation panics.
+/// Rust's formatter will panic when trying to allocate memory for very large widths.
+/// This limit is somewhat arbitrary but should be well above any practical use case
+/// while still preventing formatter panics.
+const MAX_FORMAT_WIDTH: usize = 1_000_000;
+
+/// Check if a width is too large for formatting.
+/// Returns an error if the width exceeds MAX_FORMAT_WIDTH.
+fn check_width(width: usize) -> std::io::Result<()> {
+    if width > MAX_FORMAT_WIDTH {
+        Err(std::io::Error::new(
+            std::io::ErrorKind::OutOfMemory,
+            "formatting width too large",
+        ))
+    } else {
+        Ok(())
+    }
+}
+
 /// A single item to format
 pub enum FormatItem<C: FormatChar> {
     /// A format specifier

src/uucore/src/lib/features/format/num_format.rs

@@ -693,7 +693,7 @@ fn strip_fractional_zeroes_and_dot(s: &mut String) {
 fn write_output(
     mut writer: impl Write,
     sign_indicator: String,
-    mut s: String,
+    s: String,
     width: usize,
     alignment: NumberAlignment,
 ) -> std::io::Result<()> {
@@ -706,13 +706,17 @@ fn write_output(
     // by storing remaining_width indicating the actual width needed.
     // Using min() because self.width could be 0, 0usize - 1usize should be avoided
     let remaining_width = width - min(width, sign_indicator.len());
+
+    // Check if the width is too large for formatting
+    super::check_width(remaining_width)?;
+
     match alignment {
         NumberAlignment::Left => write!(writer, "{sign_indicator}{s:<remaining_width$}"),
         NumberAlignment::RightSpace => {
             let is_sign = sign_indicator.starts_with('-') || sign_indicator.starts_with('+'); // When sign_indicator is in ['-', '+']
             if is_sign && remaining_width > 0 {
                 // Make sure sign_indicator is just next to number, e.g. "% +5.1f" 1 ==> $ +1.0
-                s = sign_indicator + s.as_str();
+                let s = sign_indicator + s.as_str();
                 write!(writer, "{s:>width$}", width = remaining_width + 1) // Since we now add sign_indicator and s together, plus 1
             } else {
                 write!(writer, "{sign_indicator}{s:>remaining_width$}")

src/uucore/src/lib/features/format/spec.rs

@@ -550,6 +550,10 @@ fn write_padded(
     left: bool,
 ) -> Result<(), FormatError> {
     let padlen = width.saturating_sub(text.len());
+
+    // Check if the padding length is too large for formatting
+    super::check_width(padlen).map_err(FormatError::IoError)?;
+
     if left {
         writer.write_all(text)?;
         write!(writer, "{: <padlen$}", "")

tests/by-util/test_printf.rs

@@ -1461,3 +1461,24 @@ fn test_emoji_formatting() {
         .succeeds()
         .stdout_only("Status: Success 🚀 🎯 Count: 42\n");
 }
+
+#[test]
+fn test_large_width_format() {
+    // Test that extremely large width specifications fail gracefully with an error
+    // rather than panicking. This tests the fix for the printf-surprise.sh GNU test.
+    // When printf tries to format with a width of 20 million, it should return
+    // an error message and exit code 1, not panic with exit code 101.
+    let test_cases = [
+        ("%20000000f", "0"),    // float formatting
+        ("%10000000s", "test"), // string formatting
+        ("%15000000d", "42"),   // integer formatting
+    ];
+
+    for (format, arg) in test_cases {
+        new_ucmd!()
+            .args(&[format, arg])
+            .fails_with_code(1)
+            .stderr_contains("write error")
+            .stdout_is("");
+    }
+}