Anti-rollback check in the bootloader

The code

Embedded C for Cortex-M. It compiles. There are 3 real bugs in it: the kind that pass a bench test and fail in the field.

typedef struct {
    uint32_t magic;
    char     version[12];            /* "major.minor.patch" */
    uint32_t size;
    uint8_t  sig[64];
} img_hdr_t;

bool boot_check(const img_hdr_t *h)
{
    if (strcmp(h->version, min_version_str()) < 0) return false;
    otp_set_min_version(h->version);
    return verify_signature(h);
}

What an interviewer expects

• Point to each line you'd stop in a code review.
• Say what actually breaks: the failure mode, not just the rule it violates.
• Fix it, then defend the fix when asked the follow-up.

The bugs, reference fixes and the follow-up question are in the practice app, after you've tried it.

Companies that test this

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