{"id":2645,"date":"2026-03-17T19:41:42","date_gmt":"2026-03-18T01:41:42","guid":{"rendered":"https:\/\/embeddedor.com\/blog\/?p=2645"},"modified":"2026-04-14T15:50:12","modified_gmt":"2026-04-14T21:50:12","slug":"here-to-fix-what-i-break-fixing-a-2-year-old-bug-in-the-linux-kernel","status":"publish","type":"post","link":"https:\/\/embeddedor.com\/blog\/2026\/03\/17\/here-to-fix-what-i-break-fixing-a-2-year-old-bug-in-the-linux-kernel\/","title":{"rendered":"Here to fix what I break: Fixing a 2-year-old bug in the Linux kernel"},"content":{"rendered":"\n
\"\"<\/figure>\n\n\n\n

<\/p>\n\n\n\n

While reviewing one<\/a> of the patches sent to the linux-hardening<\/a> list this afternoon, I spotted a bug I introduced in the same code two years ago (on March 5 2024, to be precise).

Over nearly a decade contributing to the Linux kernel, I’ve discovered and fixed many years-old bugs, and it’s always a special feeling. This time is no different. Here to fix what I break! 🙌🏼

“struct nx842_crypto_header is declared with the __packed attribute,
however the fields grouped with struct_group_tagged() were not packed.
This caused the grouped header portion of the structure to lose the
packed layout guarantees of the containing structure.

Fix this by replacing struct_group_tagged() with __struct_group(…,
…, __packed, …) so the grouped fields are packed, and the original
layout is preserved, restoring the intended packed layout of the
structure.<\/p>\n\n\n\n

Before changes:
struct nx842_crypto_header {
\tunion {
\t\tstruct {
\t\t\t__be16     magic;                \/*     0     2 *\/
\t\t\t__be16     ignore;               \/*     2     2 *\/
\t\t\tu8         groups;               \/*     4     1 *\/
\t\t};                                       \/*     0     6 *\/
\t\tstruct nx842_crypto_header_hdr hdr;      \/*     0     6 *\/
\t};                                               \/*     0     6 *\/
\tstruct nx842_crypto_header_group group[];        \/*     6     0 *\/

\t\/* size: 6, cachelines: 1, members: 2 *\/
\t\/* last cacheline: 6 bytes *\/
} __attribute__((__packed__));

After changes:
struct nx842_crypto_header {
\tunion {
\t\tstruct {
\t\t\t__be16     magic;                \/*     0     2 *\/
\t\t\t__be16     ignore;               \/*     2     2 *\/
\t\t\tu8         groups;               \/*     4     1 *\/
\t\t} __attribute__((__packed__));           \/*     0     5 *\/
\t\tstruct nx842_crypto_header_hdr hdr;      \/*     0     5 *\/
\t};                                               \/*     0     5 *\/
\tstruct nx842_crypto_header_group group[];        \/*     5     0 *\/

\t\/* size: 5, cachelines: 1, members: 2 *\/
\t\/* last cacheline: 5 bytes *\/
} __attribute__((__packed__));

\"<\/code><\/pre>\n\n\n\n
This should be applied to multiple stable trees, soon.<\/p>\n\n\n\n