Bootloader image verification

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;
    uint32_t length;
    uint8_t  sha256[32];
    uint8_t  sig[64];
} img_hdr_t;

bool image_ok(const img_hdr_t *h, const uint8_t *body)
{
    uint8_t digest[32];
    if (h->magic != IMG_MAGIC) return false;
    sha256(body, h->length, digest);
    if (memcmp(digest, h->sha256, 32) != 0) return false;
    if (ecdsa_verify(PUBKEY, h->sha256, h->sig)) return false;
    return true;
}

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

Matched on the skills each company's embedded roles emphasise.

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