gpu: nova-core: firmware: factor out an elf_str() function

Factor out a chunk of complexity into a new subroutine. This is an
incremental step in adding ELF32 support to the existing ELF64 section
support, for handling GPU firmware.

Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patch.msgid.link/20260326013902.588242-9-jhubbard@nvidia.com
[acourbot: use fuller prefix in commit message.]
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>

Author: johnhubbard

drivers/gpu/nova-core/firmware.rs

@@ -484,6 +484,13 @@ mod elf {
     // SAFETY: all bit patterns are valid for this type, and it doesn't use interior mutability.
     unsafe impl FromBytes for Elf64SHdr {}
 
+    /// Returns a NULL-terminated string from the ELF image at `offset`.
+    fn elf_str(elf: &[u8], offset: u64) -> Option<&str> {
+        let idx = usize::try_from(offset).ok()?;
+        let bytes = elf.get(idx..)?;
+        CStr::from_bytes_until_nul(bytes).ok()?.to_str().ok()
+    }
+
     /// Tries to extract section with name `name` from the ELF64 image `elf`, and returns it.
     pub(super) fn elf64_section<'a, 'b>(elf: &'a [u8], name: &'b str) -> Option<&'a [u8]> {
         let hdr = &elf
@@ -510,32 +517,15 @@ mod elf {
             .and_then(Elf64SHdr::from_bytes)?;
 
         // Find the section which name matches `name` and return it.
-        shdr.find(|&sh| {
-            let Some(hdr) = Elf64SHdr::from_bytes(sh) else {
-                return false;
-            };
-
-            let Some(name_idx) = strhdr
-                .0
-                .sh_offset
-                .checked_add(u64::from(hdr.0.sh_name))
-                .and_then(|idx| usize::try_from(idx).ok())
-            else {
-                return false;
-            };
-
-            // Get the start of the name.
-            elf.get(name_idx..)
-                .and_then(|nstr| CStr::from_bytes_until_nul(nstr).ok())
-                // Convert into str.
-                .and_then(|c_str| c_str.to_str().ok())
-                // Check that the name matches.
-                .map(|str| str == name)
-                .unwrap_or(false)
-        })
-        // Return the slice containing the section.
-        .and_then(|sh| {
+        shdr.find_map(|sh| {
             let hdr = Elf64SHdr::from_bytes(sh)?;
+            let name_offset = strhdr.0.sh_offset.checked_add(u64::from(hdr.0.sh_name))?;
+            let section_name = elf_str(elf, name_offset)?;
+
+            if section_name != name {
+                return None;
+            }
+
             let start = usize::try_from(hdr.0.sh_offset).ok()?;
             let end = usize::try_from(hdr.0.sh_size)
                 .ok()