// SPDX-License-Identifier: GPL-2.0
#include <string.h>
#include "debug.h"

#include "demangle-rust.h"

/*
 * Mangled Rust symbols look like this:
 *
 *     _$LT$std..sys..fd..FileDesc$u20$as$u20$core..ops..Drop$GT$::drop::hc68340e1baa4987a
 *
 * The original symbol is:
 *
 *     <std::sys::fd::FileDesc as core::ops::Drop>::drop
 *
 * The last component of the path is a 64-bit hash in lowercase hex, prefixed
 * with "h". Rust does not have a global namespace between crates, an illusion
 * which Rust maintains by using the hash to distinguish things that would
 * otherwise have the same symbol.
 *
 * Any path component not starting with a XID_Start character is prefixed with
 * "_".
 *
 * The following escape sequences are used:
 *
 *     ","  =>  $C$
 *     "@"  =>  $SP$
 *     "*"  =>  $BP$
 *     "&"  =>  $RF$
 *     "<"  =>  $LT$
 *     ">"  =>  $GT$
 *     "("  =>  $LP$
 *     ")"  =>  $RP$
 *     " "  =>  $u20$
 *     "'"  =>  $u27$
 *     "["  =>  $u5b$
 *     "]"  =>  $u5d$
 *     "~"  =>  $u7e$
 *
 * A double ".." means "::" and a single "." means "-".
 *
 * The only characters allowed in the mangled symbol are a-zA-Z0-9 and _.:$
 */

static const char *hash_prefix = "::h";
static const size_t hash_prefix_len = 3;
static const size_t hash_len = 16;

static bool is_prefixed_hash(const char *start);
static bool looks_like_rust(const char *sym, size_t len);
static bool unescape(const char **in, char **out, const char *seq, char value);

/*
 * INPUT:
 *     sym: symbol that has been through BFD-demangling
 *
 * This function looks for the following indicators:
 *
 *  1. The hash must consist of "h" followed by 16 lowercase hex digits.
 *
 *  2. As a sanity check, the hash must use between 5 and 15 of the 16 possible
 *     hex digits. This is true of 99.9998% of hashes so once in your life you
 *     may see a false negative. The point is to notice path components that
 *     could be Rust hashes but are probably not, like "haaaaaaaaaaaaaaaa". In
 *     this case a false positive (non-Rust symbol has an important path
 *     component removed because it looks like a Rust hash) is worse than a
 *     false negative (the rare Rust symbol is not demangled) so this sets the
 *     balance in favor of false negatives.
 *
 *  3. There must be no characters other than a-zA-Z0-9 and _.:$
 *
 *  4. There must be no unrecognized $-sign sequences.
 *
 *  5. There must be no sequence of three or more dots in a row ("...").
 */
bool
rust_is_mangled(const char *sym)
{
        size_t len, len_without_hash;

        if (!sym)
                return false;

        len = strlen(sym);
        if (len <= hash_prefix_len + hash_len)
                /* Not long enough to contain "::h" + hash + something else */
                return false;

        len_without_hash = len - (hash_prefix_len + hash_len);
        if (!is_prefixed_hash(sym + len_without_hash))
                return false;

        return looks_like_rust(sym, len_without_hash);
}

/*
 * A hash is the prefix "::h" followed by 16 lowercase hex digits. The hex
 * digits must comprise between 5 and 15 (inclusive) distinct digits.
 */
static bool is_prefixed_hash(const char *str)
{
        const char *end;
        bool seen[16];
        size_t i;
        int count;

        if (strncmp(str, hash_prefix, hash_prefix_len))
                return false;
        str += hash_prefix_len;

        memset(seen, false, sizeof(seen));
        for (end = str + hash_len; str < end; str++)
                if (*str >= '0' && *str <= '9')
                        seen[*str - '0'] = true;
                else if (*str >= 'a' && *str <= 'f')
                        seen[*str - 'a' + 10] = true;
                else
                        return false;

        /* Count how many distinct digits seen */
        count = 0;
        for (i = 0; i < 16; i++)
                if (seen[i])
                        count++;

        return count >= 5 && count <= 15;
}

static bool looks_like_rust(const char *str, size_t len)
{
        const char *end = str + len;

        while (str < end)
                switch (*str) {
                case '$':
                        if (!strncmp(str, "$C$", 3))
                                str += 3;
                        else if (!strncmp(str, "$SP$", 4)
                                        || !strncmp(str, "$BP$", 4)
                                        || !strncmp(str, "$RF$", 4)
                                        || !strncmp(str, "$LT$", 4)
                                        || !strncmp(str, "$GT$", 4)
                                        || !strncmp(str, "$LP$", 4)
                                        || !strncmp(str, "$RP$", 4))
                                str += 4;
                        else if (!strncmp(str, "$u20$", 5)
                                        || !strncmp(str, "$u27$", 5)
                                        || !strncmp(str, "$u5b$", 5)
                                        || !strncmp(str, "$u5d$", 5)
                                        || !strncmp(str, "$u7e$", 5))
                                str += 5;
                        else
                                return false;
                        break;
                case '.':
                        /* Do not allow three or more consecutive dots */
                        if (!strncmp(str, "...", 3))
                                return false;
                        /* Fall through */
                case 'a' ... 'z':
                case 'A' ... 'Z':
                case '0' ... '9':
                case '_':
                case ':':
                        str++;
                        break;
                default:
                        return false;
                }

        return true;
}

/*
 * INPUT:
 *     sym: symbol for which rust_is_mangled(sym) returns true
 *
 * The input is demangled in-place because the mangled name is always longer
 * than the demangled one.
 */
void
rust_demangle_sym(char *sym)
{
        const char *in;
        char *out;
        const char *end;

        if (!sym)
                return;

        in = sym;
        out = sym;
        end = sym + strlen(sym) - (hash_prefix_len + hash_len);

        while (in < end)
                switch (*in) {
                case '$':
                        if (!(unescape(&in, &out, "$C$", ',')
                                        || unescape(&in, &out, "$SP$", '@')
                                        || unescape(&in, &out, "$BP$", '*')
                                        || unescape(&in, &out, "$RF$", '&')
                                        || unescape(&in, &out, "$LT$", '<')
                                        || unescape(&in, &out, "$GT$", '>')
                                        || unescape(&in, &out, "$LP$", '(')
                                        || unescape(&in, &out, "$RP$", ')')
                                        || unescape(&in, &out, "$u20$", ' ')
                                        || unescape(&in, &out, "$u27$", '\'')
                                        || unescape(&in, &out, "$u5b$", '[')
                                        || unescape(&in, &out, "$u5d$", ']')
                                        || unescape(&in, &out, "$u7e$", '~'))) {
                                pr_err("demangle-rust: unexpected escape sequence");
                                goto done;
                        }
                        break;
                case '_':
                        /*
                         * If this is the start of a path component and the next
                         * character is an escape sequence, ignore the
                         * underscore. The mangler inserts an underscore to make
                         * sure the path component begins with a XID_Start
                         * character.
                         */
                        if ((in == sym || in[-1] == ':') && in[1] == '$')
                                in++;
                        else
                                *out++ = *in++;
                        break;
                case '.':
                        if (in[1] == '.') {
                                /* ".." becomes "::" */
                                *out++ = ':';
                                *out++ = ':';
                                in += 2;
                        } else {
                                /* "." becomes "-" */
                                *out++ = '-';
                                in++;
                        }
                        break;
                case 'a' ... 'z':
                case 'A' ... 'Z':
                case '0' ... '9':
                case ':':
                        *out++ = *in++;
                        break;
                default:
                        pr_err("demangle-rust: unexpected character '%c' in symbol\n",
                                *in);
                        goto done;
                }

done:
        *out = '\0';
}

static bool unescape(const char **in, char **out, const char *seq, char value)
{
        size_t len = strlen(seq);

        if (strncmp(*in, seq, len))
                return false;

        **out = value;

        *in += len;
        *out += 1;

        return true;
}