Re: [PATCH 4/8] unwind: Implement generic sframe unwinder library

Prasanna Kumar T S M
2025-01-30T10:29:16Z

On 30-01-2025 15:52, Prasanna Kumar T S M wrote:
>
> On 28-01-2025 03:03, Weinan Liu wrote:
>> This change introduces a kernel space unwinder using sframe table for
>> architectures without ORC unwinder support.
>>
>> The implementation is adapted from Josh's userspace sframe unwinder
>> proposal[1] according to the sframe v2 spec[2].
>>
>> [1]
>> https://lore.kernel.org/lkml/42c0a99236af65c09c8182e260af7bcf5aa1e158.1730150953.git.jpoimboe@kernel.org/
>> [2] https://sourceware.org/binutils/docs/sframe-spec.html
>>
>> Signed-off-by: Weinan Liu <wnliu@google.com>
>> ---
>>   include/linux/sframe_lookup.h |  43 ++++++++
>>   kernel/Makefile               |   1 +
>>   kernel/sframe_lookup.c        | 196 ++++++++++++++++++++++++++++++++++
Nit: Can this file be placed inside lib/ instead of kernel/ folder?
>>   3 files changed, 240 insertions(+)
>>   create mode 100644 include/linux/sframe_lookup.h
>>   create mode 100644 kernel/sframe_lookup.c
>>
>> diff --git a/include/linux/sframe_lookup.h
>> b/include/linux/sframe_lookup.h
>> new file mode 100644
>> index 000000000000..1c26cf1f38d4
>> --- /dev/null
>> +++ b/include/linux/sframe_lookup.h
>> @@ -0,0 +1,43 @@
>> +/* SPDX-License-Identifier: GPL-2.0 */
>> +#ifndef _LINUX_SFRAME_LOOKUP_H
>> +#define _LINUX_SFRAME_LOOKUP_H
>> +
>> +/**
>> + * struct sframe_ip_entry - sframe unwind info for given ip
>> + * @cfa_offset: Offset for the Canonical Frame Address(CFA) from Frame
>> + *              Pointer(FP) or Stack Pointer(SP)
>> + * @ra_offset: Offset for the Return Address from CFA.
>> + * @fp_offset: Offset for the Frame Pointer (FP) from CFA.
>> + * @use_fp: Use FP to get next CFA or not
>> + */
>> +struct sframe_ip_entry {
>> +    int32_t cfa_offset;
>> +    int32_t ra_offset;
>
> The ra_offset is not present for x86_64 in SFrame FRE as per the spec.
> I am wondering whether this struct should change based on the
> architecture or just set ra_offset calculated from cfa_offset for x86_64.
>
>> +    int32_t fp_offset;
>> +    bool use_fp;
>> +};
>> +
>> +/**
>> + * struct sframe_table - sframe struct of a table
>> + * @sfhdr_p: Pointer to sframe header
>> + * @fde_p: Pointer to the first of sframe frame description entry(FDE).
>> + * @fre_p: Pointer to the first of sframe frame row entry(FRE).
>> + */
>> +struct sframe_table {
>> +    struct sframe_header *sfhdr_p;
>> +    struct sframe_fde *fde_p;
>> +    char *fre_p;
>> +};
>> +
>> +#ifdef CONFIG_SFRAME_UNWINDER
>> +void init_sframe_table(void);
>> +int sframe_find_pc(unsigned long pc, struct sframe_ip_entry *entry);
>> +#else
>> +static inline void init_sframe_table(void) {}
>> +static inline int sframe_find_pc(unsigned long pc, struct
>> sframe_ip_entry *entry)
>> +{
>> +    return -EINVAL;
>> +}
>> +#endif
>> +
>> +#endif /* _LINUX_SFRAME_LOOKUP_H */
>> diff --git a/kernel/Makefile b/kernel/Makefile
>> index 87866b037fbe..705c9277e5cd 100644
>> --- a/kernel/Makefile
>> +++ b/kernel/Makefile
>> @@ -131,6 +131,7 @@ obj-$(CONFIG_WATCH_QUEUE) += watch_queue.o
>>     obj-$(CONFIG_RESOURCE_KUNIT_TEST) += resource_kunit.o
>>   obj-$(CONFIG_SYSCTL_KUNIT_TEST) += sysctl-test.o
>> +obj-$(CONFIG_SFRAME_UNWINDER) += sframe_lookup.o
>>     CFLAGS_stackleak.o += $(DISABLE_STACKLEAK_PLUGIN)
>>   obj-$(CONFIG_GCC_PLUGIN_STACKLEAK) += stackleak.o
>> diff --git a/kernel/sframe_lookup.c b/kernel/sframe_lookup.c
>> new file mode 100644
>> index 000000000000..846f1da95d89
>> --- /dev/null
>> +++ b/kernel/sframe_lookup.c
>> @@ -0,0 +1,196 @@
>> +// SPDX-License-Identifier: GPL-2.0-only
>> +#include <linux/module.h>
>> +#include <linux/sort.h>
>> +#include <linux/sframe_lookup.h>
>> +#include <linux/kallsyms.h>
>> +#include "sframe.h"
>> +
>> +#define pr_fmt(fmt)    "sframe: " fmt
>> +
>> +extern char __start_sframe_header[];
>> +extern char __stop_sframe_header[];
>> +
>> +static bool sframe_init __ro_after_init;
>> +static struct sframe_table sftbl;
>> +
>> +#define SFRAME_READ_TYPE(in, out, type)                    \
>> +({                                    \
>> +    type __tmp;                            \
>> +    memcpy(&__tmp, in, sizeof(__tmp));                \
>> +    in += sizeof(__tmp);                        \
>> +    out = __tmp;                            \
>> +})
>> +
>> +#define SFRAME_READ_ROW_ADDR(in_addr, out_addr, type)            \
>> +({                                    \
>> +    switch (type) {                            \
>> +    case SFRAME_FRE_TYPE_ADDR1:                    \
>> +        SFRAME_READ_TYPE(in_addr, out_addr, u8);        \
>> +        break;                            \
>> +    case SFRAME_FRE_TYPE_ADDR2:                    \
>> +        SFRAME_READ_TYPE(in_addr, out_addr, u16);        \
>> +        break;                            \
>> +    case SFRAME_FRE_TYPE_ADDR4:                    \
>> +        SFRAME_READ_TYPE(in_addr, out_addr, u32);        \
>> +        break;                            \
>> +    default:                            \
>> +        break;                            \
>> +    }                                \
>> +})
>> +
>> +#define SFRAME_READ_ROW_OFFSETS(in_addr, out_addr, size) \
>> +({                                    \
>> +    switch (size) {                            \
>> +    case 1:                                \
>> +        SFRAME_READ_TYPE(in_addr, out_addr, s8);        \
>> +        break;                            \
>> +    case 2:                                \
>> +        SFRAME_READ_TYPE(in_addr, out_addr, s16);        \
>> +        break;                            \
>> +    case 4:                                \
>> +        SFRAME_READ_TYPE(in_addr, out_addr, s32);        \
>> +        break;                            \
>> +    default:                            \
>> +        break;                            \
>> +    }                                \
>> +})
>> +
>> +static struct sframe_fde *find_fde(const struct sframe_table *tbl,
>> unsigned long pc)
>> +{
>> +    int l, r, m, f;
>> +    int32_t ip;
>> +    struct sframe_fde *fdep;
>> +
>> +    if (!tbl || !tbl->sfhdr_p || !tbl->fde_p)
>> +        return NULL;
>> +
>> +    ip = (pc - (unsigned long)tbl->sfhdr_p);
>> +
>> +    /* Do a binary range search to find the rightmost FDE start_addr
>> < ip */
>> +    l = m = f = 0;
>> +    r = tbl->sfhdr_p->num_fdes;
>> +    while (l < r) {
>> +        m = l + ((r - l) / 2);
>> +        fdep = tbl->fde_p + m;
>> +        if (fdep->start_addr > ip)
>> +            r = m;
>> +        else
>> +            l = m + 1;
>> +    }
>> +    /* use l - 1 because l will be the first item fdep->start_addr >
>> ip */
>> +    f = l - 1;
>> +    if (f >= tbl->sfhdr_p->num_fdes || f < 0)
>> +        return NULL;
>> +    fdep = tbl->fde_p + f;
>> +    if (ip < fdep->start_addr || ip >= fdep->start_addr + fdep->size)
>> +        return NULL;
>> +
>> +    return fdep;
>> +}
>> +
>> +static int find_fre(const struct sframe_table *tbl, unsigned long pc,
>> +        const struct sframe_fde *fdep, struct sframe_ip_entry *entry)
>> +{
>> +    int i, offset_size, offset_count;
>> +    char *fres, *offsets_loc;
>> +    int32_t ip_off;
>> +    uint32_t next_row_ip_off;
>> +    uint8_t fre_info, fde_type = SFRAME_FUNC_FDE_TYPE(fdep->info),
>> +            fre_type = SFRAME_FUNC_FRE_TYPE(fdep->info);
>> +
>> +    fres = tbl->fre_p + fdep->fres_off;
>> +
>> +    /*  Whether PCs in the FREs should be treated as masks or not */
>> +    if (fde_type == SFRAME_FDE_TYPE_PCMASK)
>> +        ip_off = pc % fdep->rep_size;
>> +    else
>> +        ip_off = (int32_t)(pc - (unsigned long)tbl->sfhdr_p) -
>> fdep->start_addr;
>> +
>> +    if (ip_off < 0 || ip_off >= fdep->size)
>> +        return -EINVAL;
>> +
>> +    /*
>> +     * FRE structure starts by address of the entry with variants
>> length. Use
>> +     * two pointers to track current head(fres) and the address of last
>> +     * offset(offsets_loc)
>> +     */
>> +    for (i = 0; i < fdep->fres_num; i++) {
>> +        SFRAME_READ_ROW_ADDR(fres, next_row_ip_off, fre_type);
>> +        if (ip_off < next_row_ip_off)
>> +            break;
>> +        SFRAME_READ_TYPE(fres, fre_info, u8);
>> +        offsets_loc = fres;
>> +        /*
>> +         * jump to the start of next fre
>> +         * fres += fre_offets_cnt*offset_size
>> +         */
>> +        fres += SFRAME_FRE_OFFSET_COUNT(fre_info) <<
>> SFRAME_FRE_OFFSET_SIZE(fre_info);
>> +    }
>> +
>> +    offset_size = 1 << SFRAME_FRE_OFFSET_SIZE(fre_info);
>> +    offset_count = SFRAME_FRE_OFFSET_COUNT(fre_info);
>> +
>> +    if (offset_count > 0) {
>> +        SFRAME_READ_ROW_OFFSETS(offsets_loc, entry->cfa_offset,
>> offset_size);
>> +        offset_count--;
>> +    }
>> +    if (offset_count > 0 && !entry->ra_offset) {
>> +        SFRAME_READ_ROW_OFFSETS(offsets_loc, entry->ra_offset,
>> offset_size);
>> +        offset_count--;
>> +    }
>> +    if (offset_count > 0 && !entry->fp_offset) {
>> +        SFRAME_READ_ROW_OFFSETS(offsets_loc, entry->fp_offset,
>> offset_size);
>> +        offset_count--;
>> +    }
>> +    if (offset_count)
>> +        return -EINVAL;
>> +
>> +    entry->use_fp = SFRAME_FRE_CFA_BASE_REG_ID(fre_info) ==
>> SFRAME_BASE_REG_FP;
>> +
>> +    return 0;
>> +}
>> +
>> +int sframe_find_pc(unsigned long pc, struct sframe_ip_entry *entry)
>> +{
>> +    struct sframe_fde *fdep;
>> +    struct sframe_table *sftbl_p = &sftbl;
>> +    int err;
>> +
>> +    if (!sframe_init)
>> +        return -EINVAL;
>> +
>> +    memset(entry, 0, sizeof(*entry));
>> +    entry->ra_offset = sftbl_p->sfhdr_p->cfa_fixed_ra_offset;
>> +    entry->fp_offset = sftbl_p->sfhdr_p->cfa_fixed_fp_offset;
>> +
>> +    fdep = find_fde(sftbl_p, pc);
>> +    if (!fdep)
>> +        return -EINVAL;
>> +    err = find_fre(sftbl_p, pc, fdep, entry);
>> +    if (err)
>> +        return err;
>> +
>> +    return 0;
>> +}
>> +
>> +void __init init_sframe_table(void)
>> +{
>> +    size_t sframe_size = (void *)__stop_sframe_header - (void
>> *)__start_sframe_header;
>> +    void *sframe_buf = __start_sframe_header;
>> +
>> +    if (sframe_size <= 0)
>> +        return;
>> +    sftbl.sfhdr_p = sframe_buf;
>> +    if (!sftbl.sfhdr_p || sftbl.sfhdr_p->preamble.magic !=
>> SFRAME_MAGIC ||
>> +        sftbl.sfhdr_p->preamble.version != SFRAME_VERSION_2 ||
>> +        !(sftbl.sfhdr_p->preamble.flags & SFRAME_F_FDE_SORTED)) {
>> +        pr_warn("WARNING: Unable to read sframe header. Disabling
>> unwinder.\n");
>> +        return;
>> +    }
>> +
>> +    sftbl.fde_p = (struct sframe_fde *)(__start_sframe_header +
>> SFRAME_HDR_SIZE(*sftbl.sfhdr_p)
>> +                        + sftbl.sfhdr_p->fdes_off);
>> +    sftbl.fre_p = __start_sframe_header +
>> SFRAME_HDR_SIZE(*sftbl.sfhdr_p)
>> +        + sftbl.sfhdr_p->fres_off;
>> +    sframe_init = true;
>> +}
> Other than the minor suggestion, the code looks good to me.
>
> Reviewed-by: Prasanna Kumar T S M <ptsm@linux.microsoft.com>.
>