[DTrace-devel] [PATCH 3/8] Add support for strchr() subroutine
Kris Van Hees
kris.van.hees at oracle.com
Thu Oct 14 12:06:14 PDT 2021
On Wed, Sep 29, 2021 at 11:13:36AM -0400, eugene.loh at oracle.com wrote:
> From: Eugene Loh <eugene.loh at oracle.com>
>
> This implementation tries to minimize branching and looping in BPF code
> to ease the load on the BPF verifier, which tries to walk each branch.
> It does so by using bpf_probe_read() to copy the string into scratch
> memory. Then, the byte in question is xor'ed with every byte in scratch
> memory. This means the matching byte will now be a NULL byte while all
> other bytes are non-null. This means that bpf_probe_read_str() can now
> give us the location of the byte in question.
>
> Signed-off-by: Eugene Loh <eugene.loh at oracle.com>
Reviewed-by: Kris Van Hees <kris.van.hees at oracle.com>
... and added to my staging dev branch. I did add the .type and .size
markers to the strchr.S file.
> ---
> bpf/Build | 1 +
> bpf/strchr.S | 146 +++++++++++++++++++++++++++++++++++++
> libdtrace/dt_cg.c | 60 ++++++++++++++-
> libdtrace/dt_dlibs.c | 1 +
> test/unittest/dif/strchr.d | 1 -
> 5 files changed, 207 insertions(+), 2 deletions(-)
> create mode 100644 bpf/strchr.S
>
> diff --git a/bpf/Build b/bpf/Build
> index 34150b20..1aa9dd62 100644
> --- a/bpf/Build
> +++ b/bpf/Build
> @@ -26,6 +26,7 @@ bpf_dlib_SOURCES = \
> get_bvar.c \
> get_tvar.c set_tvar.c \
> probe_error.c \
> + strchr.S \
> strcmp.S \
> strjoin.S \
> substr.S \
> diff --git a/bpf/strchr.S b/bpf/strchr.S
> new file mode 100644
> index 00000000..49a89ff7
> --- /dev/null
> +++ b/bpf/strchr.S
> @@ -0,0 +1,146 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Copyright (c) 2021, Oracle and/or its affiliates. All rights reserved.
> + */
> +
> +#define DT_STRLEN_BYTES 2
> +
> +#define BPF_FUNC_probe_read 4
> +#define BPF_FUNC_probe_read_str 45
> +
> +/*
> + * uint64_t dt_strchr(char *src, uint64_t c, char *dst, char *tmp) {
> + *
> + * // make a copy of the char in every byte of the register
> + * c &= 0xff;
> + * c |= (c << 8);
> + * c |= (c << 16);
> + * c |= (c << 32);
> + *
> + * // spill arguments to stack
> + * [%fp-8]=src
> + * [%fp-16]=c
> + * [%fp-24]=dst
> + * [%fp-32]=tmp
> + *
> + * // make temporary copy of string and get string length
> + * r6 = bpf_probe_read_str(dst, STRSZ, src + DT_STRLEN_BYTES);
> + * r6--;
> + *
> + * // xor the char with every byte; a match results in NULL byte
> + * r4 = roundup(r6, 8);
> + * for (r3 = 0; r3 < r4; r3 += 8)
> + * ((uint64_t *)dst)[r3] ^= c;
> + *
> + * // put a safeguard in place, then look for that NULL byte
> + * dst[r6] = '\0';
> + * r8 = bpf_probe_read_str(tmp, r6 + 1, dst);
> + * r8--;
> + *
> + * // determine length of output string
> + * r6 -= r8;
> + * if (r6 <= 0) return -1;
> + * dt_strlen_store(r6, dst);
> + *
> + * // write output string
> + * r8 += DT_STRLEN_BYTES;
> + * bpf_probe_read(dst + DT_STRLEN_BYTES, r6, src + r8);
> + * r6 += DT_STRLEN_BYTES;
> + * dst[r6] = '\0';
> + *
> + * return 0;
> + * }
> + */
> + .text
> + .align 4
> + .global dt_strchr
> +dt_strchr :
> + and %r2, 0xff /* c &= 0xff */
> + mov %r5, %r2
> + lsh %r5, 8
> + or %r2, %r5 /* c |= (c << 8) */
> + mov %r5, %r2
> + lsh %r5, 16
> + or %r2, %r5 /* c |= (c << 16) */
> + mov %r5, %r2
> + lsh %r5, 32
> + or %r2, %r5 /* c |= (c << 32) */
> +
> + stxdw [%fp+-8], %r1 /* Spill src */
> + stxdw [%fp+-16], %r2 /* Spill c */
> + stxdw [%fp+-24], %r3 /* Spill dst */
> + stxdw [%fp+-32], %r4 /* Spill tmp */
> +
> + ldxdw %r1, [%fp+-24]
> + lddw %r2, STRSZ
> + ldxdw %r3, [%fp+-8]
> + add %r3, DT_STRLEN_BYTES
> + call BPF_FUNC_probe_read_str /* r6 = bpf_probe_read_str(dst, STRSZ, src + DT_STRLEN_BYTES) */
> + mov %r6, %r0
> +
> + jsle %r6, 0, .Lerror
> +
> + sub %r6, 1 /* r6-- */
> +
> + mov %r4, %r6 /* r4 = roundup(r6, 8) */
> + add %r4, 7
> + and %r4, -8
> +
> + ldxdw %r1, [%fp+-16]
> + mov %r3, 0
> +.Lloop: /* for (r3 = 0; r3 < r4; r3 += 8) */
> + ldxdw %r5, [%fp+-24]
> + add %r5, %r3
> + ldxdw %r0, [%r5+0]
> + xor %r0, %r1 /* ((uint64_t *)dst)[r3] ^= c; */
> + stxdw [%r5+0], %r0
> + add %r3, 8
> + jlt %r3, %r4, .Lloop
> +
> + ldxdw %r2, [%fp+-24]
> + add %r2, %r6
> + mov %r0, 0
> + stxb [%r2+0], %r0 /* dst[r6] = '\0' */
> +
> + ldxdw %r1, [%fp+-32]
> + mov %r2, %r6
> + add %r2, 1
> + ldxdw %r3, [%fp+-24]
> + call BPF_FUNC_probe_read_str /* r8 = bpf_probe_read_str(tmp, r6 + 1, dst) */
> + jsle %r0, 0, .Lerror
> + lsh %r0, 32
> + arsh %r0, 32
> + mov %r8, %r0
> +
> + add %r8, -1 /* r8-- */
> +
> + sub %r6, %r8 /* r6 -= r8 */
> +
> + jsle %r6, 0, .Lerror /* if (r6 <= 0) return -1 */
> +
> + mov %r1, %r6
> + ldxdw %r2, [%fp+-24]
> + call dt_strlen_store /* dt_strlen_store(r6, dst) */
> +
> + add %r8, DT_STRLEN_BYTES /* r8 += DT_STRLEN_BYTES */
> +
> + ldxdw %r1, [%fp+-24]
> + add %r1, DT_STRLEN_BYTES
> + mov %r2, %r6
> + ldxdw %r3, [%fp+-8]
> + add %r3, %r8
> + call BPF_FUNC_probe_read /* bpf_probe_read(dst + DT_STRLEN_BYTES, r6, src + r8) */
> +
> + add %r6, DT_STRLEN_BYTES /* r6 += DT_STRLEN_BYTES */
> +
> + ldxdw %r1, [%fp+-24]
> + add %r1, %r6 /* dst[r6] = '\0' */
> + mov %r2, 0
> + stxb [%r1+0], %r2
> +
> + mov %r0, 0 /* return 0 */
> + exit
> +
> +.Lerror:
> + mov %r0, -1
> + exit
> diff --git a/libdtrace/dt_cg.c b/libdtrace/dt_cg.c
> index 01a7e9c3..7d7dcaa6 100644
> --- a/libdtrace/dt_cg.c
> +++ b/libdtrace/dt_cg.c
> @@ -3179,6 +3179,64 @@ dt_cg_subr_speculation(dt_node_t *dnp, dt_irlist_t *dlp, dt_regset_t *drp)
> TRACE_REGSET(" subr-speculation:End ");
> }
>
> +static void
> +dt_cg_subr_strchr(dt_node_t *dnp, dt_irlist_t *dlp, dt_regset_t *drp)
> +{
> + dt_ident_t *idp;
> + dt_node_t *str = dnp->dn_args;
> + dt_node_t *chr = str->dn_list;
> + uint64_t off;
> + uint_t Lfound = dt_irlist_label(dlp);
> +
> + TRACE_REGSET(" subr-strchr:Begin");
> + dt_cg_node(str, dlp, drp);
> + dt_cg_check_notnull(dlp, drp, str->dn_reg);
> + dt_cg_node(chr, dlp, drp);
> +
> + if (dt_regset_xalloc_args(drp) == -1)
> + longjmp(yypcb->pcb_jmpbuf, EDT_NOREG);
> +
> + emit(dlp, BPF_MOV_REG(BPF_REG_1, str->dn_reg));
> + dt_regset_free(drp, str->dn_reg);
> + if (str->dn_tstring)
> + dt_cg_tstring_free(yypcb, str);
> + emit(dlp, BPF_MOV_REG(BPF_REG_2, chr->dn_reg));
> + dt_regset_free(drp, chr->dn_reg);
> +
> + /*
> + * The result needs be be a temporary string, so we request one.
> + */
> + dnp->dn_reg = dt_regset_alloc(drp);
> + if (dnp->dn_reg == -1)
> + longjmp(yypcb->pcb_jmpbuf, EDT_NOREG);
> + dt_cg_tstring_alloc(yypcb, dnp);
> +
> + emit(dlp, BPF_LOAD(BPF_DW, dnp->dn_reg, BPF_REG_FP, DT_STK_DCTX));
> + emit(dlp, BPF_LOAD(BPF_DW, dnp->dn_reg, dnp->dn_reg, DCTX_MEM));
> + emit(dlp, BPF_ALU64_IMM(BPF_ADD, dnp->dn_reg, dnp->dn_tstring->dn_value));
> + emit(dlp, BPF_MOV_REG(BPF_REG_3, dnp->dn_reg));
> +
> + off = dt_cg_tstring_xalloc(yypcb);
> + emit(dlp, BPF_LOAD(BPF_DW, BPF_REG_4, BPF_REG_FP, DT_STK_DCTX));
> + emit(dlp, BPF_LOAD(BPF_DW, BPF_REG_4, BPF_REG_4, DCTX_MEM));
> + emit(dlp, BPF_ALU64_IMM(BPF_ADD, BPF_REG_4, off));
> +
> + dt_regset_xalloc(drp, BPF_REG_0);
> + idp = dt_dlib_get_func(yypcb->pcb_hdl, "dt_strchr");
> + assert(idp != NULL);
> + emite(dlp, BPF_CALL_FUNC(idp->di_id), idp);
> + dt_regset_free_args(drp);
> + dt_cg_tstring_xfree(yypcb, off);
> +
> + emit (dlp, BPF_BRANCH_IMM(BPF_JEQ, BPF_REG_0, 0, Lfound));
> + emit (dlp, BPF_MOV_IMM(dnp->dn_reg, 0));
> + emitl(dlp, Lfound,
> + BPF_NOP());
> + dt_regset_free(drp, BPF_REG_0);
> +
> + TRACE_REGSET(" subr-strchr:End ");
> +}
> +
> static void
> dt_cg_subr_strlen(dt_node_t *dnp, dt_irlist_t *dlp, dt_regset_t *drp)
> {
> @@ -3349,7 +3407,7 @@ static dt_cg_subr_f *_dt_cg_subr[DIF_SUBR_MAX + 1] = {
> [DIF_SUBR_BASENAME] = NULL,
> [DIF_SUBR_DIRNAME] = NULL,
> [DIF_SUBR_CLEANPATH] = NULL,
> - [DIF_SUBR_STRCHR] = NULL,
> + [DIF_SUBR_STRCHR] = &dt_cg_subr_strchr,
> [DIF_SUBR_STRRCHR] = NULL,
> [DIF_SUBR_STRSTR] = NULL,
> [DIF_SUBR_STRTOK] = NULL,
> diff --git a/libdtrace/dt_dlibs.c b/libdtrace/dt_dlibs.c
> index e65dc70b..c4e495e0 100644
> --- a/libdtrace/dt_dlibs.c
> +++ b/libdtrace/dt_dlibs.c
> @@ -59,6 +59,7 @@ static const dt_ident_t dt_bpf_symbols[] = {
> DT_BPF_SYMBOL(dt_get_string, DT_IDENT_SYMBOL),
> DT_BPF_SYMBOL(dt_get_tvar, DT_IDENT_SYMBOL),
> DT_BPF_SYMBOL(dt_set_tvar, DT_IDENT_SYMBOL),
> + DT_BPF_SYMBOL(dt_strchr, DT_IDENT_SYMBOL),
> DT_BPF_SYMBOL(dt_strcmp, DT_IDENT_SYMBOL),
> DT_BPF_SYMBOL(dt_strjoin, DT_IDENT_SYMBOL),
> DT_BPF_SYMBOL(dt_substr, DT_IDENT_SYMBOL),
> diff --git a/test/unittest/dif/strchr.d b/test/unittest/dif/strchr.d
> index 0a8ddbbf..d42425c6 100644
> --- a/test/unittest/dif/strchr.d
> +++ b/test/unittest/dif/strchr.d
> @@ -1,4 +1,3 @@
> -/* @@xfail: dtv2 */
> BEGIN
> {
> trace(strchr(probename, 'B'));
> --
> 2.18.4
>
>
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